Yep, a couple of water bottle caps. They provide just a bit more than enough resin for a lamp casting. And they are cheap.
The caps actually have an inner cap that are filled with the resins. Then I pour the two part resin into a larger soda liter bottle cap and mix. It took some trying different cups, but eventually I found that these water bottle caps are just the right size and plentiful around here. Lucky me? Yes sir! Lucky me!