Hello,
Does anyone know if there is an Investor website for luxury property Buyers or where one could find luxury buyers
for SFR properties over 1,000,000?
PS I hope this post is OK, if not sorry. 
Just like everything else, network in the right circles and build your buyer list. Country Clubs, estate lawyers, financial planners, luxury home builders and agents, people in the cosmetic surgery industry, etc. can be good referral sources.
Network and talk to agents, builders, and marketing reps in that market. Find out where the builders and agents market their properties. Dupont Registry and others is where you’d post the home, but marketing a luxury home is not the same as regular houses. You’d sometimes use paintings of the home, give parties, etc. http://www.luxuryhomemarketing.com/ has a luxury home marketing certification program.