I know of a company that makes good-quality heavy-duty stretched canvases (acrylic primed). They do custom sizes (along with standard sizes), so I imagine they would do the size you want.
Their email address is [email protected]
, and their phone number is 910-628-0046.
If it matters to you, you should probably ask them what shipping will cost before you order, because at that size it would have to be shipped by truck, and that could be semi-costly.