Most exchange booths can exchange pounds for dollars, what their rate is varies though. Only USD atm I know of is in the international terminal in BKK airport next to the SCB desk.
Yes, overstaying is a crime. If you don't/can't pay you'll be subject to arrest and detainment in the immigration detention facility until someone pays your fines and buys you a ticket. If you can't afford the fine, contact the US embassy in Bangkok US citizen services, they have emergency repatriation loans available.