Neil,
1953 was the first year for this factory option (it was installed at the factory on the driver's side).
Hand-made, after-market braces were sold for a while but are no longer available. Sets show up every so often on eBay. You might also try Jim Carter and Bowtie Bits for a used assembly. A fellow named MotherTrucker
mothertruckeraz@aol.com sold me a real nice 1954 assembly (you want a 53 assembly for a 47-53 bed). If you have a short bed, you'll need a special (or adapted) rear fender.
Tim