There are no originals, Fiat supply the whole ARB assembly only.
I used Superflex, Part no SF429-1816K which fitted the ARB fine, however the outside diameter was slightly larger than the originals.
I took the whole ARB assembly off the car, split the bush holding brackets, clamped up brackets with the new bushes, tightened the bolts and it's sorted.
If you're going to do the job yourself, not much point in going for the Fiat unit, it's 5 times the cost and the poly bushes will outlive the car for virtually the same amount of work.