www.ans.org used to have them, but they aren't listed as an available resource anymore. You could still email them and inquire. They were already pretty scarce when I was doing public tours in the 90s.
Having said that, searchng eBay for that specific item would be a good bet, but it may be months between sellers on something rare like that specific card AND marble combination. Another option you could pursue is to find an old card minus said marble, and attach your own marble. On eBay, there are several vendors selling large quantities of "vaseline glass" marbles containing uranium oxide, and they glow in black light. It would give added curiousity value to the intended audience (and a low low level source should you wish to demonstrate your pancake G-M probe for the audience).
Hope this helps,
Dave
