I think I will check for my undies pattern and be sure I have one. Then, stock some cotton knits and elastic.
Good idea. I've noticed the elastic pulls away from the where it is stitched to the leg opening, a lot these days.
I get upset when they twist into odd leg shapes after washing. LilDD has a few Hanes brand with one big leg, one smaller, and it irritates me to buy a (supposedly) good brand and they're cheaply made. 
I've never sewn...what's the elastic like you'd buy to make your own?
Besides the material getting thinner, I'm also having problems with the elastic in the leg openings in Hanes underwear. It doesn't even seem to be elastic (if that makes sense) anymore, but more like rubber-band material. Seriously, I wore a newer pair just a few times and now the little bit of material that didn't even completely cover the elastic in the leg openings is fully visible and looks just like a rubber-band you'd buy in the stationary aisle.
DH wears Hanes and has been complaining that not only are they much thinner, but there is less material in the fly-area.