Skinny kids

Question: Is there a place where they sell Clothes for Toddlers and Small Children that are Really small in the waist, yet long for their age? for example, here is my problem. 2 of my children are really skinny my 6 year old can fit into a 3T but its pants aren't long enough. My 3 year old can wear 18-month clothes but yet the same problem b/c they are both on the tall side yet they are skinny....Where would be the best place for me to buy clothes for them, i'm at the point where i'm willing to spend a little extra money for their clothes......HELP!