A couple of thoughts, 1) look at your oldest, ugliest, most see-through cotton fabrics, and use up some stash. 2) if you have muslin on hand, try that. 3) if you belong to a guild, or local group that does charity quilts, and the quilt will ultimately be for charity, ask if you can have some of their old, ugly, see-through fabric or muslin.
All suggestions are completely serious -- I just took over Charity Quilts for my guild, and the amount of fabric in my house easily quadrupled overnight. Some of the whites are practically see-through and would be perfect for foundations. Liz