Karen, school furniture stores carry art stools. However, the prices are higher than you might be able to find on your own. If you find the height you need, you might look at unfinished furniture stores and find stools that will work for you. How about builder's supply stores or craft/hobby stores?
You might get some ideas of prices from School Specialty's website at https://store.schoolspecialty.com .
If you put "unfinished furniture" in a search engine (I used Yahoo), you'll can find several sites that will let you compare prices on stools.
Also, how about checking with a used office furniture store? Your local elementary school(s) would probably have catalogs in the office they would let you browse through.