I suspect you can only get them from Fiat considering that motor factors can't even get hold of throttle cables :bang:
I've never had to buy one in all the years I've driven Fiats. If one failed I just got one out of a scrapyard. Might be a problem now that there are virtually no Unos in scrapyards anymore.
One thing you could do is see if the Uno cable cross references against things like early Puntos/ Cinquecentos. That might open up a few extra possibilities!