You could try Exchange & Mart. Sounds like the gearbox is knackered. They are
not things that a DIY motorist can easily strip down and repair due to the high
number of internal and special components. You might find that a Fiat dealer
can strip down the box and see if it's repairable. They did on my Cinq when a
bearing dropped out - still cost £500 and it's a bit of a gamble.
There are a number of online breakers yards who might be able to help, you'll
still have to get it fitted though.