Fuji still produces color and B&W packfilm for the 550 holder (FP100C and FP3000B).
Otherwise, Polaroid packfilms can be found on eBay from time to time. If it's B&W and has been properly stored, they last for years past their expiration date. I am currently working my way through a pile of 15 boxes of 553 (800 ISO B&W), expired 2003, with no visible ill effects.
Look at the Polaroid website and go to one of the 5xx filmpacks. Above that page you see the message "No Longer Manufactured".
They sell just wat is stored in the distribution centres.
Fuji is still producing this type of packfilms