eBay. They are straight LED's without resistors, so you can't just hook them up to your accessory output - you need to add resistance.
My solution to this is to create a 'lighting bus'. This is simply a thin, narrow material using copper tap (or somethings bare copper wire) attached to both sides.
The resistors are added to one side (I'm using 470 ohm resistors - you can wire these lights in series with lower resistors, but its actually more work to connect these tiny wires than just attached each light separately to its own resistor).
In addition to street lights, having this wire bus also makes it easy to add lighted cars. As you can see in the below photo!