Matteo,
I have not seen these thrust bearings commercially available. The one you have appears to be homemade, as the spacing between the balls is uneven.
There are two options, machine a new one that is a replica of the original using new bearing balls (rolling element bearing), or machine a trust bearing from sintered bronze without bearing bronze (plain bearing).
If you keep the bearing ball design, all that you need to have made is a new cage. The cage is not that critical, it just retains the balls from all getting loose and escaping. If you do not have the means to make the pitch diameter of the balls in the cage exactly the same as the race, be sure to allow some clearance of the balls in the holes so they can align to the correct diameter of the race. It will really depend on the surface the balls run against. If they are serviceable, then by all means stick with the balls.
If the surface the balls run against is pitted or chipped, a lot of folk have converted to a sintered bronze thrust race. These will span the pits and chips in the race. Depending on the era and model Douglas, the race may or may not have a groove for the balls. If it is a flat face, then the sintered bronze thrust washer will be the same thickness as the diameter of the balls. If the race has grooves, you can probably ignore them and have the washer run against the flat surfaces just inside and outside the groove. In that case, the thrust washer will be slightly thinner than the diameter of the balls, be slightly thicker than the original cage.
-Doug
