I had to get replacement roof rails for my 500X Cross Plus.
I got the kit below and in the kits were the listed individual part numbers.
Fiat 71807430 1 500X Accessory Roof Rail Kit (For Cross and Cross Plus)
Fiat 735615258 1 500X Accessory RHS / OS / DX Roof Rail - SUNSHINE
Fiat 735615261 1 500X Accessory LHS / NS / SX Roof Rail - SUNSHINE
Fiat 735615262 (0) 1 500X Accessory RHS / OS / DX Roof Rail Front Cap - SUNSHINE (TAPPO)
Fiat 735615265 (0) 1 500X Accessory RHS / OS / DX Roof Rail Rear Cap - SUNSHINE (TAPPO)
Fiat 735615263 (0) 1 500X Accessory LHS / NS / SX Roof Rail Front Cap - SUNSHINE (TAPPO)
Fiat 735615267 (0) 4 500X Accessory Roof Rail Plastic Plug Covers - SUNSHINE
Fiat 51906158 4 500X Accessory Bolt Roof Rail Mounting Bolts
I've not fitted mine yet but fitting will not be an issue. Each rail is secured by 4 bolts. The plastic rail end caps prise off revealing the front and back bolts. The centre plastic pop-in caps cover the centre two bolts.
The rails have felt edges that slot into the roof channel to seal the roof channel to rail edges.
Note! The colour SUNSHINE (Silver Shining) is the metallic looking dark silver/bronze finish. If you want the black then you will need a different kit.
http://www.glynhopkinpartsonline.co.uk/product/500X_Roof_Railing_Kit,_Silver_Shining_71807430
For Pop, Pop Star and Lounge Models the Kit P/N is: 7180742
http://www.glynhopkinpartsonline.co.uk/product/500X_Roof_Railing_Kit,_Black_71807422