Even the smallest 3 - 4 month tanks hold like 540 units, so semen capacity is secondary to holding time for most people just starting out.
Your best bet is probably to find a semen sales rep in your area. The ones that work for COBA or ABS (maybe others, too) are probably who you're going to need to service your tank anyway, so they'd be best to buy from. Those bigger companies are often able to offer special deals because they buy tanks in volume, and they might have some trade-ins available, or otherwise be able to hook you up with a reliable used one. If that doesn't work call one of the distributors like SEK.
You might find a used one near you on a farm sale, or craigslist, or the classifieds on this site or others. But if you don't know and trust who you're buying it from it's a crap shoot - it may hold well, or not. And the last thing you want is to lose hundreds or thousands of dollars worth of semen because of a bad tank. I'd buy a new one with a warranty if I could.