First, welcome to the Forum. This is the best place for Fiat car advice.
First port of call is often Euro Car Parts, branches everywhere, for click & collect, or mail order. Not the cheapest, but close, mostly, if local, very convenient.
http://www.eurocarparts.com/
A branch of the same organisation, same parts, but almost always cheaper is CarParts4less:
http://www.carparts4less.co.uk/
A Fiat specialist, loved by many on here, is Shop4Parts:
https://www.shop4parts.co.uk/
Another option, if you don't mind waiting a few days longer is Mister Auto:
http://www.mister-auto.co.uk/en/?gclid=CN6I-bKPrLwCFYUIwwodLB8Aqw
They show a lot of 'BOLK' branded stuff, which is apparently an own brand. Some have advised to avoid that, so with any of these, brand names are a safer bet, although a bit more expensive.
Cheap is not always a good idea. Although, no need to pay more than necessary for the same stuff. Skimping on servicing can lead to pain later.