I bought OEM Toyota mud flaps for my 2016 Limited on eBay... I have noticed a lot of dealerships have eBay accounts and sell parts for huge discounts. I paid $50 (shipped) for all 4 mud flaps from a *Toyota dealership*, which is a big discount from the ~$120 (?) MSRP. (I also have been buying other accessories from dealerships on eBay: roof cross bars, all-weather mats, etc. Huge savings for OEM items!)
Anyway, the OEM ones were pretty easy to install... probably took me an hour to do all 4, and a good chunk of that time was spent cleaning the winter sludge off the areas and learning how to remove the plastic clips without breaking them. You do have to drill 4 small holes in the liner which I was a little nervous about, but it was super easy. Some of the screws are in an awkward place if you're using a regular screwdriver, but I just put a philips bit into my socket wrench and didn't have any problems. (Didn't have to remove the wheels or anything like that.)