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All depends on how much money you want to spend. If your wanting cheap the rokinon/samyang/bower 8mm is good for about 250. There is the canon 15mm for about $500. Or you could just go all out and get the canon 8-15mm for $1500
Honestly, unless you're looking into doing a lot more photography with it, just shoot fisheye shit with your gopro... It's perfectly acceptable unless you're really trying to tie in audio.
If you really do want to get a fisheye for your T2i, do yourself a favour and absolutely avoid at all costs, anything other than the Samyang fisheye. You can get them used for around 200$. They're relatively decent, and they're made well. I used to have one, and it was good for the few times I ever really used it.
The 15mm is useless on a T2i, and the 8-15mm is one of the dumbest lenses out there to chunk 1500$ on.
That said, I'd recommend spending maybe another 100-150$ (so like 300-350$) on a Tokina 12-24mm f4 if you're really looking for a wide angle lens for your T2i. It's not as wide or fast as the 11-16, but it's a far more useful lens for the most part, and it's 200$ less than the 11-16.
Fisheyes are kinda cool for skate vids, but unless you're using a VX1000 with a deathlens, you'll never be that cool with one.