I thrifted these jeans from the boys department at Value Village last year (for >$10!) because I couldn't find a decent pair of adult ones that fit. These are in size 16 and the waist is adjustable. Score!
Whenever there is a 'basic' piece of clothing or accessory I need, I always visit the thrift store to find a version I can make changes to myself. If you've never done it before, denim jeans and shorts are one of the easiest things to thrift and DIY.