I would go at least the size of a standard sheet of steel. 99% of the material I cut is from a USA standard 48"x96" sheet, rarely from a 48"x120"sheet. Next standard size is 96"x240" but our table is not that large. Material is typically sold by the pound, most often a standard sheet is your best buy unless you find a rem/drop or something at a discount.
We offer an expedite service for a large local supplier if they need something for a customer and their tables are backed up, this actually does not pay that well or often, but in return they actually stock various sizes of sheet at our shop, which we are able to use as needed for our own products, and just call to let them know to bill us, this keeps us from needing to pay ahead and stock material ourselves.
It doesnt hurt to go do some leg work and spread the word to local fabrication shops about what services you can provide. If you do good, they will come back for more.