Awhile back, DH made me some templates out of scrap plexiglass. It worked great! :) He got the plexiglass at his job, but sometimes Lowes has scraps left where they cut a piece for someone. They'll often sell (if not give) you the scraps if you ask. Just depends on who's working at the time. ;-)
I read that someone on here used an old 3.5" floppy disk to make squares. Nifty idea!
An upside-down saucer would work for a circle, as would old CDs or DVDs. Even a Pringles tater chip lid would work for a small circle.
Glass would work, but after working with glass for several years (stained glass panels), I personally wouldn't recommend it. It's very easily broken, and can be chipped in a heartbeat by a rotary cutter bumping against it the wrong way. I've been bitten a few times too many by glass shards, if you can't tell! LOL!
Clear contact paper might work.
An old credit card or cover from an old hardcover book would give you a rectangle.
A plastic school-type ruler would work for a straightedge.
Shapes cut from plywood scraps or foam poster board would work.
Butter tub lids could be cut into shapes and used for templates. They wouldn't last forever, but should last a reasonable amount of time.
(With any thin template, watch that the rotary cutter doesn't "jump" the edge and get your fingers!)