My DH and I did this a few years ago. We just got red Hanes sweat pants and sweatshirts (from Kmart or somewhere) - and did matching socks. We had crazy blue wigs. I hand cut the "Thing 1" and "Thing 2" letters in a Seuss-y looking font out of one sheet of sticky-back white paper from the copy shop (.25 cents a sheet) and just stuck them onto the front of the sweatshirts.
We were very comfortable all night - no scratchy costumes! And afterwards, just peeled off the letters and we had/have the sweats to wear as pjs or to work around the house. Cost us less than $15 total per person.
Good luck - footed pjs can be hard to find for kids - not sure where you could find adult ones.