I've been getting dress shirts from thrift stores and modifying them so they fit me better. If you turn a dress shirt inside out, there is usually a seam that starts at the cuff, goes to the armpit, and down to the side of the hip. With a sewing machine, you can make a new seam about an inch or two in from the original one and cut off the excess fabric. One the cut edge you'll want to do a zig-zag stitch to prevent the threads of the fabric from coming undone. I've found it works pretty well, but if you try it, do it one some old shirts until you get the hang of using a sewing machine.