Those look like the middle part of the foam ring that comes with air-tites when you order them in quantity. The ones shown by Fred look like those too, so that may be the way to go.

Also, there are clear plastic spacers that casinos use in their dealer trays for games like Blackjack, etc.

Here's a source of the plastic spacers, at 25 cents each.

https://www.gamblersgeneralstore.com/store/p/863-CLEAR-CHIP-SPACER.aspx