If you don’t find it at the hardware store, any of these will work

OS Part, $1.89
http://www2.towerhobbies.com/cgi-bin...&I=LXCT54&P=ML

Fox part, 2 in package for $1.90
http://www2.towerhobbies.com/cgi-bin...?&I=LXG927&P=7

GMS nut and washer, $1.50
http://www2.towerhobbies.com/cgi-bin...?&I=LXEHG0&P=7

Super Tigre, $1.19
http://www2.towerhobbies.com/cgi-bin...?&I=LXFY78&P=7

Tower, $.99
http://www2.towerhobbies.com/cgi-bin...?&I=LXJB32&P=7


Here’s a pack of 100 nuts, that are just as good as any of the above, for less than 3 dollars
http://www.mcmaster.com/
part number 90499A805
Grade 8 Plain Steel Hex Nut 1/4"-28 Screw Size, 7/16" Width, 7/32" Height
In stock at $2.84 per Pack
This product is sold in Packs of 100