You'll have to use a little ingenuity. You can examine the Commercial Sites listing below for some commercial Web sites that sell Speed Graphics purchase a copy of Shutterbug Magazine or the Camera Shopper and peruse the ads or visit your local camera flea market. Recent prices vary widely.
Bargains are around, though.
Finally, be sure to read the Which Model pages before you decide what to buy. There are some potential pitfalls for the uninformed. Cliff Travis' All Seasons camera Equinox Photo a small, family-run outfit in Oregon that carries mostly unique items, priced to sell. See especially their Large Format section. Be sure to ask for Colin or Ben.