I got several yards of nice quility cotton muslin from Hancocks that is 108" wide for the whole cloth quilt I made and it was very nice to quilt and looks very good once quilted. If you don't want to use muslin, they have some very nice cotton fabrics that are 108" wide also.
Just a side note, when you make a whole cloth quilt, you sure do get a lot of satisfaction from what you have done.
Good luck with it.