I've seen this topic mentioned before, with little known about it. I think the best point to make is the one that MMMM made, pointing you to the eBay policy. however, search for those countries that are listed specifically in their own list and guess what? There are items for sale there from all of them.
The one thing that is strikingly different is Cuba. While I've seen many items for sale in the past, and I have a few notes from Cuba, I see there are very few offered at this time.
North Korea is often just labeled Korea, but those marked North Korea are still up. I have many from N. Korea, and I never had a problem getting some notes from there.
Same with Syria, Sudan, and Iran. All seem to be selling, but in lesser amounts than in the past.
I think the main point is that you aren't purchasing Directly From those countries or from sellers within those countries. The eBay policy is a reflection of the US Fed's policy, though, and I would think that you will find it harder to get them as time goes on. Perhaps ordering them from a neutral foreign country will allow you to get them easier.