Most of the shops (online and brick and mortar) have extra wide backing. They call it extra wide or simply quilt backing. Or you can use a sheet. If you do, do not buy an expensive high thread count sheet, it is harder to quilt. If you do need to piece, try to do something that is not a seam down the center. If making a plain backing, I will split one length in half lengthwise and attach it to the sides of the full width piece so the seams run down the sides rather than the middle. But, I rarely do this because I like to piece the backs. Whether it is simply a narrow row or two down the length or scattering in a few extra blocks. I think it just adds a bit of fun to the backside. A favorite way to 'stretch' backing is to cut it into four pieces and do something down the center and across the middle (like a cross). This is also a great way to use up all the bits and bobs left from the top.