Before I bought from glasscages, I searched around a lot and never found anyone. The best alternative I could come up with is that several LFS' around me are retailers of Oceanic and a few other manufacturers and could special order tanks that the LFS doesn't stock in the store. I will say that the prices where considerably higher than GC.
There are a number of places that sell acrylic aquariums online, though.
I should also point out the obvious reference to Craigslist. At least in my area, there are just about always a good number of tanks for sale.