I'm guessing you are talking about the DD MK12 gas block. If you want to save money and don't mind not using the real steal part there is the Dytac Vltor gas block, it has the same profile.
http://shop.ehobbyasia.com/dytac-vlt...ock-black.html
The G&P VLT gas block and the Madbull Noveske gas block are fairly close too.
If you weren't talking about the MK12 gas block ignore what I just said.