J.N.
I've heard from my muli-kid mom friends that second-hand boys pants are hard to come by when they hit about size 4/5 in the pants. Shirts are okay, but after 4 or 5, the pants get all ratty in the knees, so buying used may be difficult.
Do you know any parents of multiple kids? You'd be surprised how many keep old clothing and might be willing to give you some to help out if their kids don't fit into them anymore.
Also, if money's tight, you might want to buy a size up so the clothes last longer. Or see if a friend has a kid who's outgrown their stuff
but in addition to what's been suggested, I'd check out every single discount rack I could find. I found a couple of shirts at Carters for 50 cents a couple of weeks ago and I was amazed. Especially with it being the middle of summer, a lot of stores are trying to get rid of their summer inventory.
And on July 4th the Salvation Army is having an everything is 50% off sale -
Garage Sales! Also, on Craigslist I tend to find a lot more girls clothes than boys for sale, but it's usually after I've gone out an bought else where that someone posts what I'm looking for!
Good luck.