I see used 5-20 gallon fish tanks fairly often at second hand stores.
Value Village, Goodwill, and St. Vincent de Paul. You would want to be sure they hold water, and clean them thoroughly.
Sometimes I see smaller tanks with all the needed equipment at pet stores pretty cheap.
First question is, what kind of fish?
15 goldfish are going to need a WAY larger tank than 15 guppies.
Check your local paper. Check craigslist. Yardsales.
Post a "wanted ad". The price range I usually hear is $1/gallon if there is a stand included. $0.50/gal for just the tank. These prices are used, of course.
ALWAYS do a fill test before buying a used tank. Check carefully for any leaks or bowing of the glass. If the tank has a center brace, check that it is solid. Don't want to come home to 150gallons of water on the floor.
I'd go with Craigslist or Freecycle.org - on Freecycle, if you don't see what you're looking for you can always post a request. Whatever you get from it is FREE! If someone responds to your ad, they're not allowed to charge you for it....
Aquariums are pretty easy to come by. Check Craiglist, your local treasure hunt, newspaper, etc. Petsmart has pretty good deals usually too. Depending on the type of fish it is, you could get a ten gallon between $10 and $20. I paid $20 for a full setup with filter and everything. 20 gallons are not that much larger. Also, if you just need something temporarily to contain them, a large rubbermaid container works great and it's cheap. Again, really depends on the type of fish.