Howdy John Box!
I like the Mustang seats...even have two sets for the Valkyrie. I was able to the get the 'two up' Mustang set for the Valkyrie from a guy who bought it, rode a couple of times, didn't like it. Got it for around half price including shipping. The solo Mustang seat with back rest was purchased new from an online dealer who was clearing house. Paid less than half price for it and thought the offer was a mistake until the seat showed up at the front door. Both purchases were the result of lots of internet searches over time and some dumb luck.
The Shadow set up was purchased full price from Mustang and I think it was worth the expense. But...it is possible to get a good used Mustang seat with a little patience.
Good Luck!