Do I have to go to a main dealer

[Scottie111]

Where would be the best place to find 
1 wing mirror casing
2 door keylock plastic cover
3 roof rail end cover
4 side skirt end trim
All on 2015 multi space blu

[ItsGreen]

I'd personally price it up from the dealers... sometimes their prices can surprise you, then have a look on ebay.

[Sol]

Factory roof rail end caps are eye-watering. Something like £70 each and only come as a set of 4, I have priced them before.

Use http://www.catcar.info and put your VIN in or drill to your model for the part numbers. Can't buy anything there it's only a parts database. But then at least  you know what part number you're looking for.

Be prepared for a few hours of digging and hair pulling. This is often frustrating stuff to find. (The roof rail caps, skirt ends and trim that is. The rest is easy enough on eBay)

Look for folk breaking your model on eBay. There are always a few breaker cars available.