Not cheap but readily available - they became a standard part used on a land Rover, they are the Land Rover series I, series IIA & series III "Seat Box Lid Hasp". Going from memory the Land Rover part number is 334525.
Edit - yes it is.
Available here at GBP £10 inc tax.
Land Rover Direct original parts
334525 | Turn Buckle - Land Rover Part 334525
Another Land Rover parts supplier who has them in stock
Turnbuckle (FF012487) for Land Rover Series IIA/III