Aquacave is a sponsor and sells retro kits pretty reasonably. They also run specials frequently. You would spent as much on one of those off brand fixture as a name brand retro kit:

http://www.aquacave.com/ready-fit-48...pply-1263.html
http://www.aquacave.com/48-two-lamp-...lvue-1146.html
http://www.aquacave.com/48-tek-2-t5-...upply-695.html