The bottoms of some bottles have designs that can impress clay with interesting patterns, or that you could even use as a press mold! Some canned goods may also have patterns, although not as many.
Perhaps you're looking for a perfect circle? Bottles, cans, and lids are often great pattern-makers! Simply select the one closest to the size you want. (Remember, in case it is important to you, that the final size of your pot will be smaller. This means the impressed pattern or circle will be smaller as well.)