VHS tapes take up a lot of room, and don't store well in extreme temperatures. So people sell them off at yard sales, usually for 25 to 50 cents each.
Flea markets offer all the advantages of a yard sale, with dozens of vendors. Some even specialize in old movies.
Years of people coming in to sell their VHS tapes, coupled with decreased demand, means that most pawn shops will have tons of VHS tapes to sell.
When people have something perfectly good that they don't need anymore, it's easy to dump it off at Goodwill, the Salvation Army or other organizations that accept donations.
If want to to go online, try eBay, the Internet's worldwide garage sale. It might be the best place online to find used, or even some new, VHS tapes. You can also find people selling used VHS movies, particularly Disney movies and workout tapes, at Amazon.com.
Both sites have communities of people offering usable stuff for free.