I have gotten every single betta tank I own for free....go search yard sales and such for cheap tanks, put a post up on craigs list just asking for free tanks...you would be suprized...I got a 20 long, 2-10s, 2-5s, and 2-2s for free just by asking on craigs list. many people have old tanks sitting around in the basement or garage that they would gladly get rid of.
I have also gotten most of my equipment this way too, filters and such, all just needed a good cleaning. the only thing I have paid for is heaters.
also dont be turned off by old or leaking tanks, nearly all can be repaired...just cut off the old silicon with a razor blade and smear a good layer of aquarium safe silicon along it and presto....nice tanks for the cost of silicon.
to buy new I would have had to pay bare bones 250 dollars for my tanks and filters, and I got it all for free and saved my money for nice fish and food.