Reddit reviews Strathmore 300 Series Bristol Artist Trading Cards, Smooth Surface, 20 Sheets

Order some magic the gathering stuff from cardkingdom.com and in the special request at the end of purchase, request a token like this.

Or if you have an artistic friend, pick up some artist cards (like everything else, can be found on Amazon here) and have them do it.

You could cut out small pieces of cardstock or buy these and give your players pens/pencils to make their own tokens, that become part of the charm of your cube. all the different artworks people come up with for tokens.