I recently picked up two Zenith MC9070 audio cassette players. They both need belts. Can you still get these anywhere?

Might try here : http://www.fullcompass.com/

www.turntablebelts.com

Have used them several times, for teac, Sony decks, with good luck

On Ebay, there are assorted multi-size packs of square cassette belts, usually 30, 35, 50, or 100 belts per pack. Flat belts as well, but to a lesser extent.

You'll need to know the type (flat or square) and the circumference of the belt(s) that you need. Wrap a string around the pulleys and mark the overlapping point with ink. Measure the string and you'll have a close approximation of the circumference. Some sellers measure half the circumference ("fold long") or the diameter instead, but both can be calculated from the circumference.