1st: talk to your Instructor so you know what you are looking for. You might fnd it better to get a decent Beginner's grade set NEW, sized to fit you. It may not last as long, or it may last better, and you can get a new set for under that from several different vendors online.
2nd: If you decide you're still going with a used set, then do some research. Learn the terminology, the right sizing, what makes for good quality (materials, stitching), and where to watch for wear-and-tear (inside the palm of the gloves, padding).
3rd. Check e-Bay. And KEEP WATCHING!!!!
Use keywords like kendo armor, kumdo armor, bogu, and hogu.
I bought two complete sets, parts of a child's set, and four shinais for $400. Between all the parts, I ended up with one and a half decent sets of armor, and used shinai for any beginning student to use who visits our class.
Forgot to mention: Kendo World Magazine has a website with online forum dedicated to Japanese kendo and related arts, one section of which is devoted to equipment. Kendo and sport kumdo (Daehan Kumdo through the KKA) are essentially the same thing, and use the same armor.
So, please check out this link to their forum, and you will get to the equipment section. Notice the FAQ stickied at the top, with important information about selecting and purchasing bogu (kendo armor), ane check the other posts for information about used armor offered for sale.