3 oz plastic bathroom cups are good for just about anything that doesn't contain acetone. I used to worry about the nooks and crannies in the bottom preventing getting a good mix on epoxies, but this has turned out to never have been an issue. I finally found a dispenser to keep them in, which makes it really easy to grab one in a pinch when I need it. I think I found that dispenser on Amazon. Took me a while, as many don't dispense the smaller cups.
The other thing I find indispensable are specimen cups, also sourced from Amazon. I realized while collecting a "deposit" in one of these that they had to be a tight seal given their usual contents, and they have turned out to be great for finishes, stains and dyes. Never had one leak, they will keep most finishes fresh for at least a couple of weeks, and they're large enough to be able to dip a small foam or bristle brush into it.