for backing if you purchase a wide fabric (normally 108" up to 118" wide are the normal available) and you order 3 yds- you will have 108" long- which will give you plenty for your backing. most lqs' carry some wide fabrics- you can also get many including wide muslins at joannes. on line places like fabric.com carry a larger selection and various prices.
it can be quite a savings- figuring quilting cottons are going for $10 + a yard- if you need 8 or 9 yards= compared to 3 yds of wide fabric for $14 a yard (the prices vary- i've purchased a couple wide batiks for $18 a yard- still a huge savings.