AFAIK the following chandlers carry Wayfarer specials:
Pinnell & Bax
Boats ‘n Bits
John Parker Boats
The list goes on and on but those are the ones that seem to pop-up on top every time.
Porter’s floorboard turnbuckles today are made of aluminum 1/4 inch strip. Get some from your local DIY shop and pay pennies not pounds. If you rather want to get the old style brass turnbuckles, several of the chandlers above still stock them.
The aluminium strip is easy to cut to length with a metal saw or a hobby knife. Then drill and countersink a hole in the middle. Finally do what Porter’s always forgets, round the ends with a file. The fixed part is best made from some left over pieces of Trespa (a very hard and tough sort of plastic, available from any DIY shop). The alternative is plywood, obviously it has to be painted or varnished thoroughly. Lock the screws in place with some epoxy. The whole shaboom shouldn’t cost more then a few pennies if you are creative with left over materials. “It ain’t rocket sience” as our American friends would put it.