Hello,
I do not know where one might be obtained but one could be made fairly easily. I have a piece of rock maple that I acquired just for that purpose.
The shelf is wedge or triangular shaped in profile; right at 1" wide tapering down to a rounded 3/16" tip. The "top" (where your arrow will rest)will be crescent or half-moon shaped fitted to the contour of the riser and 5/16" wide at its midpoint. The top of the wedge is also crowned and angled slightly towards the riser. The wedge tapers in thickness from top to bottom from 5/16" to 1/16" or less at the tip of the wedge.
These measurements are from what I believe to an original rest on one of my grizzlies. I also have one that has a rest made from a piece of elk antler.
I hope this helps and that I did not make it sound more complicated than it is.
Take Care,
Tom