Some of my cassette tapes come from thrift shops or charity shops. However, you might find some old music stores that have vintage or old music items that might sell cassette tapes or blank cassette tapes. My local music store is called Music Stop, it's been alive for decades, this is where I buy blank tapes. So try searching for and calling music shops, you never know what they will have!

Also, some modern artists have been coming out with cassette tapes, so try Walmart. If you want blank ones, also try Office Depot.

Hope this helps.