I have the majority of the NOW Comics TRG series. I have two copies of the first several issues because I noticed some of them have Slimer in the lower corner of the cover while the others have the UPC code. I just scouted eBay for a while until I got all of mine, and I didn't pay much for them. They're also in excellent condition. I had many of them already, minus the first 3 issues, but all of mine had seen a lot of wear over the years.
I also have an annual or two, the Ghostbusters II movie adaptation (thought in bad shape), the GB role playing game (very cool), and the little books sold in the UK based on the RGB cartoon, including Drool, the Dog-Faced Goblin, Mrs. Rogers Neighborhood, and many others. Those are really cool, so keep an eye out for them.