I usually get cork rounds at reptile shows, but who knows when those will be happening again. Chewy.com sells medium cork rounds from ZooMed, and they're currently in stock (I just got a few, actually). Pangea sells cork rounds in assorted sizes that you can choose from, and Dart Frog Connection has a couple of sizes but the COVID19 situation has affected shipping and supplies, and neither has them in stock right now. Glass Box Tropicals has cork rounds, but I think they are small. From your photo, it looks like you already found New England Herpetoculture - and like the other companies, their shipping options are limited right now. I think you'll be able to get what you need online, but unfortunately it looks like it won't happen just yet.
If you google 'cork bark planters' a few places have them but good lord, they're expensive.