Neopets actually had TCG tournaments IRL as well as on the site and people that were into TCG games actually played. There were even staff tournaments and some coincided with several plots I think. That said, sellers on Amazon and probably Ebay still sell cards. I bought a set hoping to get a redeemable code but that was a bust. Even though the seller advertised it as "brand new" and "unopened original packaging" when I received it, the cellophane wrapper was opened and resealed and the virtual prize code was missing. I've bookmarked some others on Amazon but I'm not eager to buy since I can't trust the descriptions of "New and Unopened". I am trying to collect all twelve hundred or so TCG cards though since you do have a TCG album on Neopets that you can still fill. Problem is, beyond buying form folks selling them in their shops or on the TP and potentially a random event that gives you one, I don't know how else to get TCG cards now. They used to be given out when you played Hubrid's Hero Heist (not sure if you still can get cards from it though; I should play ), through virtual prize codes and when TCG was a thing on the site, through the game itself. There are also 7 avatars related to the TCG era (most are now unavailable though as they were also tied to plots and tournaments).
You can check out Jellyneo's article on it here: http://www.jellyneo.net/?go=tcg . Happy Hunting!