The LED's are not too bad to come by but you gotta be careful which RGB you get as there are 2 types:
RGB with 3 different color diodes (will not create "white" light) RGB with 1 single diode.. This one creates "white" with all lit (and they do cost more)
All are available on Ebay at about the best prices I could find.. Just takes time to get them from CH.
These will work directly off a 2, 3, or 4S battery but the 4S will likely shorten the life of the diodes.
Waterproof versions weigh in at about double the non.. If you care careful, you can pull the silicone waterproof layer off the strip however.. just takes a bit of time and patience..
The big 10W I got online somewhere but here is pretty much the same one (seems about the same cost as well.. if you dig, you will find the thing out there and maybe even cheaper.. (do post up if you do find a better deal)