So this is why people search for alternative Polaroid 600 instant film. There is only one alternative you have in market and that's Impossible Project 600 film. They make three types of old Polaroid film with those machinery they could salvage and bought from closed down Polaroid factories. Even those they can produce film for only Type 600, SX-70 and Spectra cameras. If you have one of these instant cameras then you are in luck. Because you can buy film for these cameras.
They have started making film for Polaroid 600 cameras several years back and last year they launched more updated type of film from their first product. They have Polaroid 600 color film and also Polaroid 600 black and white film. Also you can buy it with several different types of frames.
Impossible project film comes in 8 sheet film packs. These film packs cost you around $25 and that means each photo print cost you around $3. You can buy these from local camera shops if available but not many willing to keep this kind of film anymore. If you buy from Impossible project site it cost you around $24 plus shipping fee. From Amazon you can get it for around $25 - $27 with free shipping so it might be the cheapest offer you can find.