Most laser printers should be able to print onto transparency sheets.
Inkjet is a little more problematic because most inkjets are water-based, which obviously doesn't work well when sprayed on a sheet of plastic. But they do make transparency film specially for use with inkjet printers; see your local office supply store.
Given a printer and transparency, then all you have to do is print a hex grid, perhaps getting a PDF with your favorite sizes, colors, etc from
Incompetech. (Now, if we can just talk them into heavier-weight megahex outlines...)