Every tractor dealer has a bone yard out back somewhere. You will need to drive around and go see what they have. Don't rely on asking someone to check the inventory. If the dealer is computerized, chances are a 40 year old sickle mower is not in the computer inventory. My new dealer just relocated across town to a huge new site. They still brought about 15 old pieces with them and created a new bone yard at the new site.
I bought a JD #39-N sickle mower in decent shape from my old dealer. It works fine, but could use some new paint and blades. I bought a complete set of blades for about $20 and still have to install them when I get a few spare hours. It was $600 and I gladly paid it after searching for about four months for one. Used working sickle mowers are hard to find under $750. I believe mine is about 40 years old. The belt looks about 20 years old, but has not broken yet.
Make sure to get a manual for it so you can understand all the adjustable features. They are actually more complex than they first appear. They are hard to find because the typical buyer today is using them to maintain private roads, pond edges, etc and not to actually cut hay with. The rule of thumb is that it will cut any tree that will fit between the fingers protruding beyond the blades. There is tremendous power in those blades when 20-40HP is pushing them. Manuals are available from e-bay or other TBN members
IMPORTANT SAFETY NOTE: Before you attempt to move the sickle bar up or down, make sure someone shows you how to lift it properly. If you do it wrong, you will have one or more of your fingers cut off in less than a second.