Mylar sheets (technically biaxially-oriented polyester film) can be purchased from various online marketplaces, specialized craft stores, and industrial plastic suppliers. The best place to buy depends on whether you need them for stencils, drafting, insulation, or food storage.

These sites offer the widest range of pre-cut sheet sizes (like 12"x12" or 12"x24") and thickness options (typically 4 mil to 14 mil).

30 Pack 4 Mil Blank Stencil Sheets, 12 x 12inch Clear ... - Amazon.com