Any holster that will fit a S&W 'K" frame will fit the Ruger Security Six.
You might haunt e-bay - I see some good bargains on lightly used holsters and some used police rigs, including full flap. Of course, Left Hand rigs are a bit more scarce but on the other hand, there is less demand so they tend to sell for less.
Just my $0.02 worth.
Terry T