Pretty much anything that's on the Cartel Market can be found on the GTN. Some of it of course costs quite a lot of credits, but that's mostly for rare pieces that aren't necessarily better than cheap stuff. Then again, I never bothered buying gear before I got to 50, saved me quite a bit of money, I think.
I was going to try to add mods to some of my own equipment but the work table kept saying i needed some kind of enhahnacemnt slot to add stuff and i couldnt find it for sale anywhere so skilled that too.
I didn't quite get that. Are you talking about the Augment slot? If so, yes, the augment slot needs to be manually added to gear with an augment kit. If you're talking about the Enhancement slot... It's absent on some gear, I think, but never on custom gear, so look for that.