(This is a frequently asked question)
What you need is a Fuji PA-145 or Polaroid 405 film holder. These take the smaller pack Fuji Instax film, which is widely available.
That's all you have for instant film choice these days. The Fuji PA-45 and Polaroid 550 holders are for the now-discontinued 4x5 pack film. The 545 holder is for the Polaroid sheet film.