If you ever find anything let me know!! I have the worst possible time trying to find pants for DS13 -- he is SUPER skinny (I think it was 85 pounds at his last appointment) 90% of the size 16 pants do NOT have adjustable waists anymore and once you go into the WxL jeans they most certainly don't. I have no idea what I'm going to do after size 16 gets too short. He has a belt but doesn't care to wear it too much & even if he did, there is still a lot of fabric to attempt to get bunched up. He may have no choice pretty soon.
I buy Size 16 slim with adjustable waists and pull them in whenever I can find them but they are HARD to find. I have found 16 Regulars & 16 Huskies but 16 Slims are few & far between as I will look on the shelf & see they don't even have a spot for them -- add in the need for adjustable waist and ugh.
When I DO find them, I try to snag as many pairs as I can but I don't want to go overboard because I know he will grow out of them lengthwise. I think Meijers is where I found some and Target. I know Wal-Mart around here anyway doesn't hardly carry any slims.