Where on the Tesla website can I find the cost of a vegan upholstery option?
You mean cloth of vinyl?
If you are speaking specifically for the Model 3, there is ordering information for that car yet. Also, anything you do find on the MS/MX sites, can't be used as a basis for the Model 3 because no one outside of Tesla knows what the costs are for anything related to the Model 3.
I suspect it'll be like the S where non-leather upholstery is included in the base price. Is it vegan? I don't know but I'd guess yes.