Check out Jamestown Distributers, they handle it. They have recently come out with a version which does NOT require sanding between coats. Haven't used it but sounds like something I might use. I used 4 1000 ml cans on my DB, not cheap but I don't think you'll find a better varnish.
You'll need to refresh the varnish every two-three years. Just give it a light wet sanding with 180-220, slap on a couple of coats and you'll be good to go.
Here's a picture last year after I added a couple of coats (8-9 total) to refresh my cedar strip canoe.
I took one of Brad Dimock's boatbuilding classes a couple of years ago - he had an instuctor from the Wooden Boat School in Maine there. They both recommended the Pettit varnishes - also available from Jamestown.