Two wedges against a baton secured to a joist etc work just as well and IMO better than a dedicated (and sometimes expensive) floorboard cramp. The lightweight pressed steel models cannot provide the force needed unless you are lucky to have arrow straight boards. Axminster, Screwfix and Rutlands stock various models.
What they said. But if you're wanting an old, old-fashioned one for some reason (and you might, who knows?), you could try asking the dealers listed here, or the ubiquitous Ebay of course :roll: