I think any way. TBBvol. 1 under backings mentioned a elusive "clarified calf skin" possibly carried buy suppliers for musical instruments makers.
Vellum, traditional vellum, that ancient documents were written on, was thinned raw hide. Same stuff is used for drums today. Not the paper under the same name.
Google lead me to a company that sells calf vellum,as well as goat and sheep. Calf comes in two colors, white and, clear. Most are the size of books....but 1/2 and whole hides are available. For the price I think it might be easier to just get goat or deer raw hide.
Just wanted to throw that out there.