Just ordered that Officer Cartman plush from eBay. the leather on the shoes was peeling and the "As Seen In 'Chickenlover' " tag is still on, and I paid $40 for him.
I started in 1998 when the world was a different place. If you are wanting to go old school and have the money go for the Fun-4-All plushies. If you don't have the money I'd go for the newer Kidrobot plushies at Spencer's or the Kidrobot website. There are many options but these days it is hard to find things for very cheap, especially the older stuff. So it is hard to say where to look besides Mercari and eBay. I still browse those sites sometimes and I keep up to date on Spencer's, Hot Topic, Kidrobot and YouTooz websites. Some collector's get inside and tips and deals with others but I am not one of them and I tend to not buy from big collector's. It is very easy to get ripped off. Believe me, I have MANY times. Some friendly collectors have more experience in what prices should be so it's always good to make friends with collectors who can help you out. The good ones usually tell you what they're really worth opposed to what people are charging. Anyway, that is my two cents!