I got one from Randy a couple years ago for a buddy that saw mine, not sure if he still makes them.
Randy's gauge is a little different from the Sinclair gauge, with Randy's all the measurements are from the muzzle, the Sinclair tool takes them from the other end by removing the bolt and using a action guide.
With Randy's you measure right to the bolt face and tip of the bullet,
With the Sinclair tool you measure to the base of the bullet and the base of the cartridge.
I like Randy's version better but I'm sure they both work