One handy resource for tape deck belts, aside from the library of service manuals at hifiengine (whose user registration has been "under maintenance" for months, at least), is turntableneedles.com.
Despite the domain name they carry a large variety of belts for things well beyond turntables, and more importantly have a searchable resource to match belts to product models and OEM service manual numbers: https://www.turntableneedles.com/find10.asp
If they don't stock the belt, you can often find the same-sized belt elsewhere. But it helps immensely to know what you're looking for. They also stock tools designed for belt replacements. This helped me replace the belts in my old Realistic/Radio Shack boombox's deck.