Mirror caps are an OEM Seat part, part number 6J0 072 500.
They're also available in a brushed aluminium finish, part number 6J0 072 500A.
Easy to fit, they replace the original body colour mirror caps and just push on and clip in place.
I picked mine up off eBay second hand but like new for about £25, they're around £50 new from the dealers.
If you're looking for a set online these are listed to fit certain
Ibiza & Leon as well, may get more results searching for them than for Exeo.