As Dawn mentioned, Trumbull Mountain is a great resource. They have instructions for obtaining correct tracings on their website. You send those in to them and Nancy at Trumbull will suggest various saddles, and ship you saddles that she feels would be an appropriate fit.
Another good resource (also in Vermont!

is Colleen Meyer at Advanced Saddle Fit. Just google for their website -- it is a wealth of information. She is a master saddler and is wonderful to talk to. She is also making her own saddle now, which is very similar to the Albion, but about $1,000 less.
It is important to remember that there is much more to saddle fit than tree size. Tree shape, placement of stirrup bars, type of panels, gussetts, etc., all play a large role.
And as Tam said, one saddlers Medium might be another's wide. (I have a Sommer for sale that is stamped as a wide, but it is a 36cm tree. For other manufacturers, that would be called a XW or perhaps even an XXW.) So, it is important to go on actual measurements, and not just the medium, MW, or W tree designations!