Another option is to put a WTB add up on SR25.com and see what you come up with. If you don't already have a Mk11 or a gas block, getting the spare gas block can be really problematic.
You can try ordering one through the ADS catalog. I haven't bought one, but I called and they quoted me 1,800-2,200 back in 11/08. I can't remember the exact quote.