Kegsystems ask $19.50 for a replacement one. Try your local HBS as most will have some parts lying around. I got some from my LHBS for $5 each. Make sure you take your keg in and try the fitting on because it gets really confusing and it is sometimes near impossible to distiguish Firestone/Spartanburg from Cornelius. Note, they are not interchangable.
Good luck, but you should be right.
Justin