Without doing any research, I bought mine from Bimmain for my E46 M3. These LEDs are very bright but not at all easy to fit into the existing light bulb housing.
Later I found out there are at least two types of LEDs replacement, 1) there is the "bulb" type which looks a bit like the original license plate bulb and should be plug and play, and 2) the bigger ones which incorporate the LED circuit board and the light bulb cover into one single unit. They are meant to replace the original license plate light cover.
I have the latter but I could not fit it in with the cover the comes with the LED. I had to modify the original cover in order to snap it in and it was still a tight fit.