1/4-20 prop nut: http://www2.towerhobbies.com/cgi-bin...?&I=LXCT54&P=Z
Washer: http://www2.towerhobbies.com/cgi-bin...?&I=LXCT71&P=Z

Like the other guys have said, be careful buying something at the hardware store. Most of what you'll find there is very soft. The threads in the nut will distort or strip and the washer will bend. This could possibly lead to the prop coming off and hurting somebody. If you have a fastener supplier or hardware store in your area with a good selection, you may find a hardened nut and washer but I wouldn't try it unless you were absolutely sure.