Reddit mentions of GoldenTrading Mixed Square Cassette Tape Machine Recorder Rubber Belt for Repair Maintenance - Black (Packs of 30)

eBay is usually the place to go for that kind of thing. Unfortunately, I cannot find round VCR belts on eBay anymore, for whatever reason (used to be able to buy them in this massive bags that contained a variety of sizes). You can get a pack of square belts from Amazon, which may provide an awkward fit, but it might still work. Since I haven't bought this pack specifically, I cannot vouch for the quality of their construction, nor their ability to fit to a gear they may not be meant for.

https://www.amazon.com/GoldenTrading-Cassette-Machine-Recorder-Maintenance/dp/B01CZSU6K8/ref=lp_172550_1_6?s=aht&ie=UTF8&qid=1571595620&sr=1-6

Alternatively, if you want the long way around, you can try to get the belt out of the camera (as long as it hasn't melted, which happens a lot), measure it's diameter, and buy a single belt of that specific length from eBay.

My suggestion is to roll the dice on a big bag'o belts like this one
Or a set of belts on ebay for a similar JVC from around the same time.
Or, if you can find the parts list for that boombox, locate the JVC part number for the belts. That's what I ended up doing for my boombox; once I had the part number I was able to find it on some online hi-fi part store that came up in a google search. Good luck!

Well you can take off the current drive belt and measure it, typically they are sized in mm. When I was fixing VCRs back in the day, Napa o-ring gasket material worked well. Try Amazon; they’ve got kits of square drive belts etc in mm increments.

GoldenTrading Mixed Square Cassette Tape Machine Recorder Rubber Belt for Repair Maintenance - Black (Packs of 30) https://www.amazon.com/dp/B01CZSU6K8/ref=cm_sw_r_cp_api_i_KlEBDbNND91PR

Checking the ION site they have parts for their turntables but not for your gear. May want to contact them direct to either get new part or at least specs. Mileage may vary. Good luck.