Original Polaroid 545-compatible sheet instant film is no longer in regular production; Polaroid-made film for that holder ended years ago. Based on the answers, the main current/new option mentioned is New55, which produced 4x5 peel-apart film yielding both a positive and a negative. Another option mentioned is OneInstant, but that is for smaller peel-apart formats, not true 4x5 sheet film for a 545.

Fuji FP-100C was pack film, not the same format as Polaroid 545 sheet film, so it does not directly solve 545 compatibility.

If you specifically want to use the 545 holder, availability is extremely limited and effectively depends on specialty products rather than mainstream film. If no fresh 545-compatible film is available, your alternatives are to use expired stock at your own risk, or experiment with adapting other 4x5 materials in the darkroom—but that would be a DIY workaround, not a standard supported instant-film solution.