Volpin Props' Harrison Krix is my main prop building inspiration. This guy makes beautiful props. BUT he also made a vacuum form machine.

While I was searching for industry standard sizes in plastic sheets I came acros many sites that sell many different sizes in styrene sheets. For example: kunststofshop.nl This is a good resource for bigger sheets of styrene, acrylic, PVC and more. Sheets start at 500x500mm and have a variety of ticknesses.

For smaller sheets I found modelbouwshopnederland.nl Smaller sheets are better for high detail objects and miniatures.

Of course there is also plastic modelkit standard sheets by Evergreenscalemodels.com which has sizes in inches. 6" x 12" is what they have most. (152,4mm x 304,8)

Build-stuff.com has very nice ready to go plans for a vacuumformer. It's quite an industrial machine considering it's all metal and steel.