Garage sales, maybe. Goodwill, forget about it! I haven't seen a good tape deck pop up at Goodwill in years, and when one does, it's either beat up and broken with a high price tag or a cheap budget unit that was sold back in the day. Side note: Goodwill continues to sell DVD and Blu-Ray players with no remotes, making the unit practically useless you can obtain a replacement remote, or a universal remote that mostly works, which can be another 10-20 dollars.
Belts are usually the cause of inoperable tape decks. Some are easy to replace, others require a bit of disassembly. Other issues can include dirty heads, broken plastic gears, bad capstans, and oxidized contacts.