If you're looking for a particular fabric, consider buying a discounted RTW garment a size or two bigger than yours, and recut the fabric. With many items you could also reuse some of the plackets, fly fronts, zippers or buttons. I was at the mall the other day, and many of the stores had serious sales on. I was looking for pants, and I saw a lot marked down to below $20 that had originally been priced three or four times as much. And these were decent colors and sizes, not just bright orange in sizes 2 and 4. I saw nice RPL pants for $18; you couldn't buy good RPL yardage for that. (I also got a nice black pencil skirt for an additional 40% off, because the zipper was broken. Yahoo!)
I've done this with some Goodwill garments in the past and it's really not much more fiddly than using yardage (and it keeps clothes out of the landfill). I try to remind myself that clothes are just fabric, and if there's enough fabric I could turn them into something else entirely if I wanted.
The Slapdash Sewist has a nice blog entry up on making an oversized sweater into a sweater dress; it's super-cute.