For practice, I get all those pesky little leftover pieces of batting and zigzag them together. My sister uses old bath towels. And I buy plain old muslin by the bolt, usually waiting for it to go on sale or use a 40-50% coupon at JoAnn's. Muslin is also great for making practice blocks, especially if there are a lot of pieces to it and/or I've never done the block before. However, when I am going to FMQ a quilt, I use the same fabric and batting as the quilt to practice on so that I can make any adjustments to the machine.