Not as hard to come by as you might think. Pre unit Triumph Trophys through late '56 used a 20" rim with 40 holes. Also the BSA B50T and the B50MX, as well as the B25T and the T25T Triumph (which was really a BSA with a Triumph badge) I think, used a 40 hole 20" rim. Those last bikes are from the early '70's. Someone on this board offered me a complete wheel from one of those just the other day. I will pass on to him that you are looking
Although it's not for sale, I scored a flanged alloy 40 hole 20" rim somewhere. It seems too light to be a Vincent rim, and from the weight and dimples looks like a SanRemo brand. You may already know this but every once in a while Avon tools up and makes a batch of 3.00 x 20 Speedmaster MKIIs. I have one to go on that rim.