I ordered a seat post from rbikes. The attached link should help, and there are some inexpensive ones.

http://rbikes.com/page.cfm?PageID=27...62&affiliate=2

I would recommend finding another shop to help you measure. Free Spirit is low end bike, and the bike shop that you went to wasn't really interested in helping you out. You should find a bike shop that's more service oriented - they're giving you some passive attitude, so I would not go back to them for repairs or anything else. If you measure accross the tube - RBikes may be able to help you over the phone - but shipping may be as much as a cheap post, finding a local used bike place is better.