You can piece batting. Here's the method I prefer.
Overlap 2 pieces of batting. Cut a wavy line through both with a rotary cutter (maybe 4 to 6 inch curves). Discard the 2 wavy ends. The remaining pieces of batting will fit together perfectly. Use a tailor's hand tacking stitch on each side of the cut to secure the 2 pieces together. (Google tailor tacking, or watch Susan Schamber's video on how she bastes her quilt sandwich to see how that stitch is done.)
The reason for making the wavy cuts through both is so you don't end up with a "crease" in your quilt where the batting joins.