Most home centers do have it but don't know it. Plastic laminate comes in two thicknesses. For horizontal applications there is 1/16". For vertical applications there is 1/32". If you ask there someone may know. They do have damaged sheets, that they are happy to let go cheap.
You could call some local cabinet shops, as they use it regularly. Look up in your yellow pages for "plastic laminate", or it could be listed under "plastics - sheets, rods, tubes".
Also in the yellow pages look up "cabinet makers supplies". There are usually local suppliers of materials to cabinet shops, like those that sell sheet goods like plywood. If nothing else turns out well, call a few cabinet shops and ask where you could buy the material, or if they would sell you a half sheet.
You could go online to any of the major brands: Pionite, Wilsonart, Lamin-Art, Formica, Nevamar, Micarta, and use their locator by input of your zip code in the locator box.