Bomber Pro, Shorter, Iro MVP, Some old british steel thing
crest on metropolitan near lorimer in williamsburg is one of the best in town, but even they don't have a huge metric stock. An auto supply store sometimes is the way to go. Not sure how Lowe's and Home Depot do in the metric stuff but they are both close to your home...
When I worked as a General Contractor, I always went to Lendy. Lendy is a very strange hardware store that has everything you could ever want. You step into the store, get a number, tell the guy at the counter what you want, and he goes into the warehouse to get it for you. Trust me, this place has everything. I think it's located off Broome & Mott..... where little Italy and Chinatown merge.