I am starting to find this with my boys. The older they get, the harder it becomes to find gently used clothing. I have now started asking friends with older boys for hand-me downs (I offer to buy from them). A friend up the street has a 9 year old boy and she set aside 3-4 pair of pants/jeans, 3-4 shirts and a couple sweaters for my 7 year old. I know they are in excellent condition so my son has "nice" things. And she wouldn't let me pay for them!!! Then, I get hand me downs from my nephew which may be a bit more worn but I cull through and look for the best things. Right now, my son has more clothes than he needs and I have only purchased 2 pair of jeans!
Asking friends with older sons (and family) is how I've done it for the past year. Then I can hand these down to my 5 year old in a couple of years.
Also, I have found that consignment shops have better quality merchandise than our area thrift/goodwill stores (IMHO). Once Upon a Child is a chain that I really do use a lot. You pay a bit more than 25 cents for an item but for things that both my boys can wear, I'm ok paying $3 for a pair of jeans (but there are expensive items I wouldn't buy like $10 for "designer" jeans!). Plus, if you consign with them, you can get free clothes plus they have great sales if you are on the email list. I just took 3 tubs of DD clothes. They told me it was worth $90. Did I want that in a check or merchandise or both? So I shopped and picked up jeans for my oldest, boots for my DD, 2 jackets for my middle son (who will only wear "hoodies") and some toys and books for gifts. I still walked out with $60+ and a bag of great items.
good luck! It's expensive trying to keep growing boys in nice clothes but it can be done!