For MDM-7 variant drives, I've been using the 17mm belts from eBay seller chn_03 (Charles Holt) in the UK. I've repaired 3 decks (MDS-JE440, MDS-JE480, MDS-E10) with these recently. (Charlie was very helpful and sold me a multi-pack of just the 17mm size).
https://www.ebay.co.uk/itm/17mm-Rubber-Drive-Belt-for-Cassette-Deck-Player-CD-ROM-DVD-Player-etc/192420535089
It's about a 5 minute job to replace these:
Unplug
Remove lid (4 side screws, maybe 1 rear screw)
Remove the drive from the deck
4 mount screws
Two ribbon cables from the main PCB
Maybe a chassis earth in some units to unclip from the main chassis
Use tweezers to lift off the vertical tensioner spring on the left (drive opening facing you) - careful to not let it ping away!!!
Ease apart the vertical slider at the rear, left gently and slide to the left the upper part of the disk load frame - and you're in for an easy belt change.
Refit is the reverse, keeping the ribbon cables square as you gently reinsert into their sockets on the main PCB.