A local framing shop should be able to order you as big as 48x96 in 4 or 8 ply. I could give you the name of my framer, but you really want someone local who can order from the wholesaler and have it delivered locally.
Because oversize mat boards (and museum glass) are special orders, you may have to order several sheets in one order.
I think the reason is that it requires special packaging and handling for even one mat to keep it flat and not bent or crunched. Thus shipping costs soon add up for it to be prohibitive for a single mat.
However, a local framer might have one on-hand or be willing to let you piggyback one of his orders.
