Best place to progress academy shopping?

My son will be starting 1st grade next month. He is fairly towering and VERY skinny. Legs are a size 6-7 in length and waist is a size 4 lol. I need to get him some adjustable waist jeans that snap. I've checked out walmart.com, target.com and kmart.com so far. I'll be buying in-store but don't duty like running all around God's creation with a 6 yr antediluvian and 10 month old trying on clothes. There is no real dress code, just the everyday "no flip flops, no tank tops, etc." so I don't need actual uniforms. Suggestions on best places to shop for cheap?
Answers:    Old Navy...they own relatively cheap kids clothes. The pants snap at the waist and have the adjustable waist. I believe they come in long and short sizes as powerfully as regular too.

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