If you're going for the period look, however, you can make your farthingale out of silk, as is specified by Alcega in his Instructions on making a farthingale. Queen Elizabeth had farthingales made out of satin and taffeta. You can also make it out of a sturdy linen. There's no record of farthingales made out of brocade or velvet, but bridal satin and shot-silk taffeta, both reproductions of the original silk-based fabrics, can also be found at your local fabric store. If you want to make it out of silk, you can find a number of very strong and sturdy silk weaves at Thai Silks, listed on the Mailorder Supplies list.
The average farthingale takes around three yards of 45" wide fabric. For a very wide one, 4 yards should be enough.
A friend of mine has used timber strapping to stiffen her farthingale. Although it is rather wide, it works splendidly once the ends have been filed round and smooth. It is very springy, and can hold out the heaviest of velvet skirts.
Period two boning materiels were used to stiffen farthingales: whalebone, and bent rope. Whalebone is unavailable these days, but can be substituted with artificial whalebone, or hoopskirt boning, for a material of similar strength and flatness. Bent rope was a rope made of bent, or reed grass. It too is unavailable these days. You can make your own bent rope by buying bundles of tiny, 00 (double-ought) round reed, taking 20 pieces of reed, and binding them with linen cord or artificial sinew to create round boning 5/8 an inch in diameter. Bent rope is lighter than artificial whalebone or hoopskirt boning, is surprisingly flexible, yet holds its shape well. Here's a few places to look:
Plymouth Reed and Cane Supply English Basketry Willows 1200 W. Ann Arbor Rd RFD 1, Box 124A Plymouth, MI 48170 South New Berlin, NY 13843-9649 Phone: (313) 455-2150 Phone: (607) 847-8264 W.H. Kilby & Co. Ltd. 1840 Davenport Road Toronto, ON, Canada M6N 1B7 Phone: (416) 656-1065 Fax: (416) 656-1700