I save all sorts of plastic containers for use in the garden, and also metal tuna cans etc.
Hmm, does a tuna/chicken/whatever small tin hold enough or about the same of seed starting mix a 5 oz. paper cup holds? I don't have such a can or I would measure it myself. That has to be enough mix to feed the sprouting seeds until they are big enough to remove from the cup/tin.
Maybe I should ask this question in regard to this:
At what point do you take seedlings out of a 5 oz. paper cup? I may be screwing that up and leaving them in too long or not long enough.