Old Frost-Free Sillcock Woes
I would not try to find parts if you have acces to the connections.  I would convert to pex then install a new frost free faucet. 

Replacement if you know EXACTLY what parts to buy, with the proper tools on hand, and a single trip to the parts store should take around an hour.

Fixing your existing frostfree, locating parts, turning the main on and off, ect could take days if you have to wait on parts after you spend hours trying to find them.  Further at the end of it all you STILL might need to replace the faucet.

Your time has to be worth something right?

Without knowing what to buy and without the proper tools, you are probably looking double to triple the time for either option.

If the connections are not accessible; everything might change.

If you don't like to do what it takes to get to the connections (to dirty, to cramped, etc), pay someone else to and make money to pay them doing something more preferable.