The same part was used in
Fiat Punto I, for back seats. The only difference was an element positioning the belt, which (in Barchetta version) is removable and may be taken from your existing seatbelt.
Does not matter if you buy left or right, they are exchangeable.
Do not order belts for front seats of Punto I as they may be equipped with pretensioners and you do not want that (pretensioners are located at buckle socket in Barchetta, not at the seatbelt mechanism as in Punto I).
You may also browse adds / auctions for other Fiat models to find the same part, if it looks the same it will fit fine too
And you are, of course, aware that the seatbelts in B will never retract as smooth and fast as in other cars? This is due to sharp turn that the belt makes behind the seat, just before its mechanism. There were same third party solutions for that, in form of extra roll or bracket, but I have newer seen it in action. Anyone knows where to buy those?