I'm pretty sure it's a Vauxhall/GM gearbox, and the good news is that it's very common. You should be able to get one from a Vectra that's been broken for parts.
It just needs someone with more knowledge than me to confirm, then lots of treble-checking of part numbers to make sure it's the same.
Fiat will only refer to their own part numbers, but if you have a look at the thing itself you may well find a GM part number moulded into it. If so then that's what you need to get one at a more sensible price.