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.