K.B.
I don't know how high they go to, and the price can sometimes be a bit much, but my son is very slim and tall for a 6 year old. He wears 7-8's but if we buy regular I can literally fit two fist between him and his pants but they are the correct length. I got to Children's Place. It's the ONLY place I have found so far that I can get pants that fit him without a belt so they don't fall off. (He has fine motor skill issues and can't do belts yet on his own). We buy slim sizes. Most sweat pants/warm up pants fit him, so we don't have a problem there. I baught Reebock warm ups from Kohl's and he LOVES them because they are soft and have a matching hoody! They typically have decent prices. I take coupons in b/c we always have them. (And my mother in law swears by name brand stuff and helps with the cost :-) ) They also have a lot of options in pull on shirts. I found that their clothes were similar in style to anywhere else included L.L.Bean which we have bought from in past years.
Underwear, we have an issue with that too. I have found after a few months of washing that the size he's in doesn't fit but brand new ones in the same size fit perfect. The next size up is still HUGE on him.
Good luck shopping!