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Which bar are you referring to?
I left out the aftermarket sway bar part of the answer. I know of no one making them. But you should be able to find one as it is pretty unusual for them to break. There are some large salvage yards in Arizona and Texas that have a lot of 50's car and should be able to provide that for you.
Here are a couple
3C Salvage 330-262-9300
FREMAN'S AUTO (406) 287-5436
An article with several listed.
You would be surprised, there are a lot of those old cars in yards that do not have that many miles on them, so if you want one you can use as is it is quite possible to find them.
I gave you info to several yards, and I would make some calls and see what you can find.
There is no one I know of rebuilding those as a specialty. You would just need to find a machine shop or fabrication shop to do it for you.
1957 Plymouth Fury 8 Cylinders 5.2L 2BL I know of 2 places to check for used parts, http://www.car-part.com/ and http://www.junkyarddog.com/ the second boasts about having a database with over 5,100 Used Auto Junk Yards, this is the largest list of dealers of used auto parts ( used auto spares ) on the net.
Let me see what I can find.
I found one at car-part for $50.
1957Stabilizer Bar OnlyPlymouth
All American Classics USA-WA(Vancouver) E-mail 1-800-955-4999
Vancouver, WA 98682