I agree with Tim, and I dont think it would be that much of a price increase.
Lets say you buy three yards of 2.5oz. Depending on how tall you are, this would give you close to a yard of double insulated area, probably as much as you want? $29.85 from thru-hiker
The alternative is probably to get 1.5 yards of 2.5oz and 1 yard of 5oz, overlapping a little where they meet and sew together. $28.88 from thru-hiker
But thru-hiker is the only place I know of off hand that sells 5oz XP, and they only take orders for full yards.