Legwork. While I've found four of my machines on Craig's list, two of my machines have come to me "by way of mouth", (I make it well known that I want old machines...not necessarily that I COLLECT old machines, that tends to raise the price!) and the others have all come by hitting the thrift stores and antique stores.
Try putting an ad on Craig's List that you're looking for a machine, leave out the words "vintage" and/or "antique", and be specific. Example:
"Wanted: Older sewing machine for personal use. Prefer older Viking, Singer, White, New Home. Willing to pay up to $75 (or however much your budget is). "