The last available part no. for these models you describe was 159056G for 3 belts and 159056A for 25 belts, but this went out of print recently.
You'll have to buy current part no. 301291 (301291G for 3 belts, or 301291A for 25 belts), however, you will also need to stretch it out with heat (douse it in warm water) to make it work right with the older 500s.
I'm not sure, but found these online. Maybe someone more knowledgeable can confirm or deny if this is what you need.
Yellow stripe kirby vacuum cleaner belts. Found some for sale on Ebay 10 for $14.15
And a few stores online have them