Three Sisters Fabrics will send up to 5 backing samples for $1.50 (that's a total of $1.50, not per sample, and no cost for shipping). The samples are 5" squares.
http://www.threesistersfabric.com/fa...and-muslins-9/
Thousands of Bolts also has samples available for many of their backing fabrics, but they're $0.99 each.
http://www.thousandsofbolts.com/quiltbacks.html