The rear wing is not a part that can be taken off and a new one put on, as it is an integral part of the car and the same panel runs the entire length of the car at the top and bottom.
The best thing to do would be to give it to a decent bodyshop who will knock the dent(s) back in slightly then fill till it's flat, then blend the paint in respraying up to the top of the panel and down to the sill. You will be able to see if you look *really* hard (I've not seen one yet that I can't tell) but without knowing it's there you'd NEVER spot it. I did similar with rattle cans on my mate's fiesta last summer, and you couldn't tell unless you knew where to look (pretty lucky with the paint match! Cheap car only worth £400 so not worth a professional job). A bodyshop would give a much more professional result.
I'd budget around £250, but I'm pretty out of touch with prices as I get mates' rates on anything I need done professionally