Typical MTB rims are 559, and 650b is 584. The difference in brake shoe position is 12.5 mm. If canti brakes (you have canti posts, I assume you are going to use them) can accommodate this mismatch, you probably don't need a new fork. I'm putting together my first canti-based bike, and I'm not sure at this point if they can be so flexible. Or you can remove the canti posts and go to a big brake like a vintage Mafac Raid or a Tektro Bigmouth. Those two are usually designed to clear big fenders as well as the big rubber donut.
If you do want the new fork, I had a new 700c low-trail fork made by a builder in Michigan, Ralph Ellis at Ypsilanti Cycles, for a very reasonable price. Well less than the US masters, such as Weigle, but I chose to use blades he had on hand rather than buy more expensive Kaisei blades. But the design is what I wanted, the shape is pretty, and it has brazeons where they needed to be.
Last edited by Road Fan; 03-10-12 at 07:04 AM.