Claudia will be teaching a class at this years Summit on Fabric Sources, but in the meantime, let me suggest some words to Google.
Min Fab , Carr Textile, cut yardage textiles. Others here may give you specific links.
There are no shortage of fabric jobbers, though you'll want to be very specific in your quizzing of these companies before paying for books.
Personally, I only ever use cotton 8 ounces per yard or more, and I only ever order from folks who actually have stock.
You'll want to know what you are willing to sell, as 99% of what books they may ask you to buy will be appropriate for windows but perhaps not slipcovers.