Places like Ross, TJMaxx, Marshalls, or Tuesday Morning often have some of the best deals around, but you'll need to be cautious about size. These are closeout deals, and sometimes they stock luggage that is now too big to fly without paying the oversize charge. (They can be heavy, too. Try to stick with pieces that weigh no more than 10 lbs. empty.)
ALWAYS bring a soft measuring tape with you when you go shopping for luggage, as the measurements on the label are often smaller than reality. The standard limit for checked bags on most US carriers is a total size (L + W + H) of 62". This INCLUDES the wheels and handles. In most cases if the bag expands, anything over a stated 27" height will be too large when expanded.
I'll roll the bag over to the linens dept and fill it with towels in order to check the size packed (putting the towels back neatly, of course), and with books, in order to check how well it rolls when it is fully packed. Duffels, in particular, have a tendency to "grow" quite a lot when filled up, and can go over limit very easily.
For carryons, most US carriers currently limit to no more than 22" (height). However, size allowances for carryons have been dropping elsewhere in the world. If there is any chance that you might need to use it to fly outside the US, I'd stick to no more than 19" tall if at all possible.