It is unlikely that any normal factor will list or stock such a thing, as it is not a fast moving service item. Choices would be Fiat dealer, or a bearing specialist.
Bearings usually have markings on them, a series of numbers, sometimes with letters, that designate their dimensions and type. It is very rare for bearings not to have these.
There are bearings specialists around that can match any bearing you need, from the markings. This does of course mean you need to extract it before ordering its replacement.
You will of course need seals as well, input shaft, and both drive shaft seals (left and right are different). It appears that the input shaft bearing is the last thing to come out when dismantling the box, so it would be best to replace all the bearings.
Seals also usually have markings, and can often be obtained from bearing specialists, but the Fiat dealer might be best for these.