Definitely not the dealer if you're after a cost effective replacement as dealer price will be in primer and you'd have the added cost of getting them sprayed.
Your best best it to keep an eye out on eBay, GumTree etc.
Alternatively search for 'Seat Leon
Cupra breaking' as well as 'Seat Leon
Cupra side skirts' and message a few of the sellers and see if they have them available still.
I wouldn't want to pay more then £50-£75 for a pair it I'm honest. You might find that it will be easier to buy as a pair then individually and maybe sell the one you have left or keep as a spare.
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