Dimmers
Often you may want to include a dimmer in the lamp, so people can adjust the lighting levels to suit their use of the space. This used to be simple when lights were incandescent or halogen, but LED lamps cannot be used with a dimmer for incandescent lamps - the lamp may flicker or be damaged.
Also, not all LED bulbs can be dimmed - the ones that can will show a dimmable logo and label on their packaging. But you can use a non-dimmable bulb with a dimmable switch, just by setting the brightness to 100%.
For mains powered bulbs, there are two dimming technologies for LEDs: leading or trailing edge. Leading edge dimmers came first, but these sometimes make a buzzing sound when dimmed. The newer trailing edge dimmers don't suffer nearly as much from this. But many bulbs are designed to work with just one type of dimmer, and won't work with the other. Better quality dimmers are "universal" - either the dimming mode can be switched manually, or the dimmer selects the right mode automatically.
Dimmers will typically be specified with two power ranges (in Watts). The higher range, for incandescent and halogen lamps, just requires that the power of the bulb(s) on the circuit is less than that of the dimmer. The lower range may be something like 4W - 20W, and it is important that the bulbs fall within this range, so if the range is 4W - 20W then a 3W bulb should not be used.
As with all things electrical, there are standards that dimmers need to comply with: IEC (or EN) 61058-1 and 61058-2-1. If you're considering a dimmer that isn't from a major manufacturer, it's worth asking for their Declaration of Conformity to check this. If they can't provide one you can still use the component, but then the onus in on you to be able to ensure compliance.
When selecting a dimmer, a soft start will improve bulb life - bulb brightness is ramped up over a second. And overload protection prevents damage to the dimmer if too powerful a bulb is inserted - generally switching the bulb off. Also, some lower quality dimmers don't actually switch the bulb off, but dim it to a level where there is no light but there is still a current flowing through the system.
LED bulbs may tend to flicker if dimmed down too far - few can go below 10%. Some dimmers simply avoid this by setting a conservative maximum dimming value - sometimes as high as 40%. Others have an adjustable screw or similar that you can use to optimise the dimming, but this is one more thing for the customer to have to understand.
Because the dimming characteristics vary with both the model of dimmer used, and the bulb model, it makes sense to recommend a bulb to be used with your lamp to get the behaviour you want. Some bulbs may dim evenly as the dimmer is adjusted, whereas others may dim much more quickly at the start or end; and some may dim down to a lower light level than others.
I have not seen a mains dimmer fitted in the bulb holder; they are included in the switch fitted into the cord. Very few of the dimmers available are designed to accommodate an earth wire, so if having a good choice of dimmer is important to you then you need to design the lamp to be double insulated.
Two things I haven't touched on here. You can get a number of "smart" bulbs that you can control from an app on your phone or via tools like Alexa - these are simple alternative bulbs, so have minimal effect on the lamp design. And, of course, you can build the transformer and dimmer into the lamp, but then this means a lot more complications, regulations, and electrical skills than I'm covering.
Inline Dimmers
There are many inline dimmers on eBay and Amazon. Just about all of these are trailing edge dimmers, but labelled "Universal" because they can also dim CFL, halogen and incandescent lamps. Also, most are for Class II lamps, with no provision for an earth wire - you can normally work around this by running the unbroken earth wire to the side or under the PCB, but this is a pain and takes time.
Here are some offerings from major manufacturers:
Arditi are an Italian company, selling through LampSpares and probably others. Their dimmers are all trailing edge Class II, either just for LEDs or "Universal" to cover a variety of bulb types.
Relco are another Italian company with a good range of dimmers. All the in-line ones I've looked at are Class II trailing edge. They sell online and also through various resellers including Superlec and LampMasters.
Ehrmann in Germany do a range of dimmers that can be Trailing edge, Leading edge or Universal (i.e. both leading and trailing edge), though they are a bit more expensive than the others. They also have a 15 page document of tests of many leading lightbulbs against their dimmers, which shows how important it is to match the dimmer and the bulb to ensure good results. A resellers is RS Components.
Looking at those on Amazon and eBay, there is a large variety in quality, and most are 2-wire trailing edge dimmers. VIPMOON tend to be good quality, and some of their latest ones both including earth wiring, and also have an adjuster screw inside to set the minimum lighting output, so you can avoid flickering of the bulb or it going out before the dimmer is turned right down. Another way of choosing is to look at the PCB manufacturer, as many of the dimmer manufacturers just buy in standard PCBs and design their own casing round it. You can often see the PCB in images of the opened up dimmer. Tremelec are used by VIPMOON and other quality manufacturers.
Videos
Modifying a Dimmer to have Dimmed and Non-Dimmed Outputs
For a lamp that has one bulb non-dimmable and another dimmable.
Comparison of eBay Cheapie and Quality Dimmers
This disassembles versions of each and delves into the electrical difference, so some electrical knowledge is required to get the most from the video.
Electrical Comparison of Leading and Trailing Edge Dimmers
Looking at wave forms on an oscilloscope - again some electrical knowledge helps.
Comparison of Dimmers
This compares a number of wall mounted dimmer switches for electric installers, and doesn't g into the electrics as deeply as the above videos.