if you're in the st augustine florida
area, by far the cheapest prices i've ever seen for sunbrella are at sailors exchange. i paid $5 a yard for a roll of about 40 yards. i only needed 20 yards but the couple i had do the measuring/sewing for interior cushions
took the other 20 yards as partial payment for their work. they used it to do the interior
of their boat.
they sell 'remnants'. it's genuine sunbrella. the problem is they only sell an entire roll. the rolls, being remnants, are various lengths and various colors/patterns. given the high cost of shipping
you probably have to go there and look around - they have a pretty large inventory.
they do have a website of sorts but i'm not sure it would be of much help to you.