I'm guessing since you seem to do a lot of research on this subject, you probably checked out all the various websites such as: Autotempest.com, Cargurus.com, autotrader.com, etc. You should also just check out websites of local high end dealership in your general area - e.g. Ferrari/Maserati, Lamborghini, Rolls Royce, Bentley, etc. - as they often may have McLarens in their inventory that they simply don't list on those websites. I know I found a few nice comparables this way. Also, I would go to perhaps your local McLaren dealership and ask if they have any used in stock or ask if they know of any either selling privately or in any nearby dealerships. Sometimes they will tell you these sort of things.
Anyway just my two cents. I wish you the very best of luck on your search!!! May you find the right car and may the right car find you!
Enjoy life but be reasonable, you never know but you might actually live another day to pay for it!