I have a headboard with built in lighting. For some reason the manufacturer chose to use E17 intermediate lamp sockets. The selection of E17 lamps is poor; Ikea used to carry a bunch, including a good CFL, but no more.
I later read that the use of E17 sockets has become popular due to a provision in the Energy Independence and Security Act of 2007 that exempts specialty bulbs from the phase-out of incandescent bulbs, though the headboard I have is older than 2007.
There are now a bunch of adapters available to change E11 or E17 to something else (E11 to E17 or GU10, E11 to E27 or GU10, etc. (as well as adapters to go the other way), but going from small to large adds too much length for some applications.
The fixtures I have are not really intended for E27 bulbs because there would no longer be a reflector if I used the larger sockets, so I decided to change the E17 sockets to GU10 sockets, where there is a wide selection of incandescent, fluorescent, and LED bulbs available, all with built-in reflectors.
Of course I could not find GU10 sockets at a big box hardware store, so I ordered them from Amazon, 6 for $10.99 and they came the next day. They require 4-40 bolts to mount so I went off to my local electronics surplus store which has a very wide selection of 4-40 hardware at very low prices. I had to use some spacers to bring the socket out so the bulb would not be too recessed.
I only had 35W GU10 lamps at home and SWMBO said that they were too bright. So next I ordered some LED GU10 lamps from Amazon .
I wish that the ceiling fan and light fixture manufacturers would move to GU10. The integrated reflector simplifies the design, and allows for different beam angles as well as allowing for lower wattage lamps that provide the same illumination as higher wattage not MR lamps because of the matched reflector. Also, the GU10 LED lamps tend to be much less expensive than the E27 LED lamps.