Recent Shred Stixx use Futures fins. The cheapest rail fins I've seen at retail are the SB1's. Foam-EZ lists them for @ $17, here.
(plus tax and shipping). I do believe that SS is using the trailer fin from the SB3, which is 3.25" deep.
I would try calling the folks at Shred Stixx or TXShreds directly, no doubt they have replacement fins for cheap. Failing that, FoamEZ, The Surfboard Warehouse and others carry futures fins. The SB1's are 3.5" deep and get the job done, if not much else. They are rail fins only, however.
Also, I've been on ebay and if you do a search for futures fins, I've seen some crazy cheap. Look for a tri fin, or thruster set (three fins) and fins should be no more than 4" deep, unless you want a very stable ride, then deeper fins are fine. Many folks prefer the tail to be "looser", with less drive, in that case you'll want shallower fins, 3.25" depth, or you can remove the center trailer fin.
Hope that helps!