Shot Glasses- Not Just For Bad Decision Enhancement Anymore

The biggest problem with paper mixing cups is that, well, they're paper. They don't hold liquids for long unless they're the waxed variety. Water will eventually soak through, and solvents like mineral spirits or acetone will begin seeping through almost immediately. But if you're mixing resin, you have to avoid the wax coated cups, as the vigorous mixing required usually scrapes bits of the wax off into your resin. Not to mention, the minute or two of mixing with some epoxies can cause the cup to fall apart entirely.
Shot glasses are a great replacement. If you hunt them down at thrift stores, they can be had for less than a buck, and sometimes you can find a bag of several for only $2 or $3. Which, while admittedly about equal in price to a whole package of paper cups, they are reusable, which means they'll be cheaper in the long run. Once the paint is dries, just throw them into a bucket of hot water to soak for a bit, and the paint will usually peel right off.

Even resin can be cleaned out of them with a little effort. Resins stick to the glass pretty tenaciously, but it's mostly a vacuum seal and not a true chemical or mechanical adhesion. Getting a quarter inch thick wafer out of the bottom can be trying, but if you poured out the excess before it sets, the resin will usually just peel right off. (If you're really thinking ahead, you can pour out the excess, wipe it out with a paper towel, and then clean the rest using the appropriate solvent.)

Try to find glasses with a smooth, rounded transition between the wall of the glass and the bottom. Unmixed resin always hides in sharp corners, so rounded ones make mixing easier. You can also find little votive candle holders, which are about the same size (and price) as shot glasses, but with vertical sides rather than tapered.

Even better, if you can find them, are the little tiny glass pitchers that some restaurants use to serve individual portions of syrup with pancakes. They're a little bigger than a shot glass, but they're usually the same price, and the little pour spout makes pouring resin or paint much easier. They're a little harder to come by than shot glasses, but if you happen across one or two, snap them up.