I just bought custom fit F150 towing mirrors. By "custom fit," I simply mean that they were specifically designed for the F150. If you can't find those for the Toyota Highlander, then you have to go with a universal fit mirror.
I bought
one of these to use with a Ford Explorer. It really worked great for 5 years. It wasn't perfect, of course, but I liked it and it was durable.
I liked how it mounted, too.