Yeah, it gets really thin for boys pants after age 7 or so -- they just wear them out. This is really the only article of clothing for our whole family that we have a hard time finding in thrift stores. I agree with LiveLean, though, that most boys don't seem to need/want much.
We are lucky because one of our local thrift stores keeps a rack of school uniform pants -- plain khaki and plain navy blue pants -- all sizes for $1 (!) Unlike regular boys' pants, those don't get worn out, and are all in perfect condition. Good brands too -- Lands End, LL Bean, etc. DS -- 9 years old -- tried them on and liked them, so we bought four pair. He wears the navy ones for playing (navy is great for hiding grass stains) and the khaki ones when he wants to look nicer.
Other than these trousers, I will probably buy him a pair of jeans and a pair of sweat pants at Target, and call it good for the year.