I've had good luck with
Atomic Avenue.
I filled in a lot of my TMNT back issues through them. Only thing is I'm not sure but you might have to be registered with ComicBase since it's their site, but they do have a free unlimited trial plan, which I use and it's very fast and easy to set up. It lets you keep track of up to 500 comics, which is just about the perfect size to track my TMNT collection (until it gets a little bigger, then I'll have to pay for an upgrade)