I don't know if you can get the spares parts you are after, but a while back I had a nice old leather cover I wanted to keep, but the rest of the fixings were broken, so I just bought a compete new repro saddle of the same size and robbed the bits I needed from that.
The 'two pronged' fasteners are called 'bifurcated rivets' and should be available from a good vehicle upholsterers, or, if you do a 'bifurcated rivets' search for them on ebay there are loads....I doubt the metal clips are a standard upholsterers fitting but made for the job. A few years back when I had a batch of saddle covers made I got those laser cut in stainless steel..in a small quantity I guess you will have to make them by hand....Ian
Any sheet metal fabricator should be able to cut you a thin strip of steel and punch the holes in the ends for you, but i'd be careful of using stainless, steel as it may be a bit springy to get nice folds in it.