There is a NOS one on Ebay that is not too badly priced. I'll see if I can find it and post the link for you.
you could try this website( wemausa) .If you don"t mind non stock. put one of these in my 47 ford all stainless it has a sliding float. about 75.00 with shipping.the co. is in florida
I forgot I have autometer gauges in the 47. you could call them (954-463-1075) the ohm range is 240-33 which is standard for a lot of aftermarket gauges.my part # 151320. what youdo is measure the depth of your tank and subtract an inch and they make it for you. it is all stainless and 5 hole pattern. hope this helps.
I was looking hard last year and couldn't find anything that would work. I just found this from a link in another post. I have not confirmed if it actually works with the 56 gauge yet. http://www.autocitycl***ic.com/1952-1956/1955-1956-ford/mercury-sending-unit-f31/