Where can i get this part? It is the vacuum diaghram with the chain that connects to the throttle linkage.
It totally depends on what year of cruise control you're looking for. Anything post 1969 would have a transducer unit that has a speedo cable coming out of it and that would be connected to a diaphragm and a chain, or, if it's pre-69 it's hooked up to a "brain" unit under the dash which also has another component built into the speedo. The diaphragm outside would then be driven off vacuum. And then to complicate matters even more, Skylark with the Super Turbine 300 used something totally different again.
Look on ebay and in junk yards. If you don't care about originality, you can often make fit one from another car. The original one on mine was torn when I first bought it and I think I was able to get one from an 80's Caddy to fit close enough that it worked.