Can anyone recommend a good vendor of small tins? Buying them at the
supermarket gets expensive, and I hate wasting mints (I already have
too many! my teeth can't take anymore!). For my current project even a
box half the size of an Altoids box would be too big (I am currently
looking for something the size of a matchbox, or round ones as big
around as a quarter and about 1/3" deep).

Any recommendations would be much appreciated!

Go visit your local gunshop. In particular, the ones that cater to the black
powder folks. The primers for a muzzle loader come in exactly your
description of the "round one". After they've used the primers up, they're
thrown away. Might visit a compounding pharmacy as well. A lot of medication
salves come that way too, the pharmacist no doubt has empties after
compounding some medication or other or perhaps even has empties that he
stocks to supply customers with some of his products.

There's good reason to modularize, but this is getting close to too much
modularization.

W4ZCB