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.
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There are a number of online breakers yards who might be able to help, you'll still have to get it fitted though.
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Peter