Pre-cut liners are convenient, but they turn out to be expensive in the long run.
Look for bun pan liners. They are available in 2 sizes that I know of, half sheet and full sheet. I cut them down to the sizes I need.
Oh, and you don't have to line the entire bottom of the pan. I cut my liner to about 2/3 the size of the pan bottom. Unless you burn the heck out of it, that is all you need.