The brand "Tolex" no longer exists to identify the vinyl covering that was then employed by amps manufacturers as well as for auto tops up to I believe the 80s. Though today you can still get them from a few US suppliers that may have sourced them from OEMs, possibly China.
The costs for a yard runs from about $13 to $20/30 per yard plus shipping to wherever. It's been 3 years since I last ordered, I don't recall the exact prices. The prices depend on the texture that you need i.e. bronco, elephant, rough, smooth, tweed, Fender style, Vox style, British style.
I have used them to re-'tolex' my Fender Rhodes which entails quite more dererity vis-a-vis a nore straight forward amp box.
I've checked Shell Canvass then, but their vinyl covering materials (locally known as 'leatherette') is geared for the softer sofa/seating use and will not stand against the handling abuse on amps.