The best source I have found is Irvington Music near Portland. Their website is irvmusic.com. They normally have a couple hundred prerecorded tapes for sale, mostly classical, but some others. Their inventory is very large, and they rotate their offerings. Almost all of them are 7.5 ips 4tr They also feature some master dubs which are typically 15ips 2 track. They accept returns if the tape is defective. If you get on their mailing list, then you get their new offerings earlier than they post on their website.
Two track is rare and expensive, but as a rule of thumb better than 4tr. I avoid the slow speed 3 3/4 ips.
Larry