If the problem is just a small pinhole, theres many fixes for it. Por15 makes a kit, JB weld would do, and I'm sure there are others. If it's a big hole, either get a good used tank or fix yours. If you look at posts on the poly tank on this board and others, there is a vapor leak problem. Not something I would like in cab, kinda scary.
Sounds stupid, but check marine supply houses for quality boat tanks. My friend replaced the tank in his 40 with a poly boat tank and he gets no vapors and he picked up exactly 1 gallon of capacity over stock. It fits in the hole pretty perfectly (I recall he had to space the cover up with 2 washers) but other than that required very little work.) I also think it was only 40 bucks but i could be wrong about that.