craigslist, or try asking around with gymnasts or at gyms if they know of any old ones not being used. for a gymnastics one, you will most likely find a frame and then have to buy the rest
in terms of whats good enough: it would be awesome to find a cheap gymnastics one, but you really dont need one. find a friend with a tramp you like (obv a really bouncy one), look up the company name, and buy it. if thats still too much, go for craigslist