If I am struggling I usually try a Google search, and also look on ebay. Between them they usually provide pointers. Once armed with the part number you can Google that and it will usually lead to after,market options.
Shop4Parts are a really good firm dealing mainly in Fiat spares you can reach them on shop4parts.cp.uk . S4P will deliver the next day. Its a family run business and the chap I deal with is called Mick. Hes great and may also be able to help. S4P have a big range of genuine Fiat Spares. Wth filters I prefer the ones actually specified by Fiat.
Another good Place is Alfa Spares who can also often help. Use decent quality parts like Magneti Marelli and you wont go far wrong.