It's a different kind of spool, longer (70mm!) and with much bigger flanges in order to hold 5m of film instead of about 0.8m. They're normally in the canisters because there's no reason to separate them. Not available separately - they just come with the canisters.

Canisters in good condition with spool usually sell for $6-$15 each on eBay, they seem to come up once every week or two. And if you buy film in units of 15' to 20', it comes in a canister ready to use and you keep the canister for reloading and reuse.

You may even get lucky and score a canister or two when you buy your back (A70).