Alden LHS Sizing?

I bet this question has been asked a million times by now, but I do need some advice from guys with experience with Alden's. I want to get shell, but this store sadly did not carry any.

I went into a store today and tried on some calf LHS's, and I was measured with a Brannock device, and the size came out to be around a 9.5C. I really hate heel slip, and a size 9.0D was slipping on my heel but wasn't completely loose. The shoe also felt well-fitted around the heel and around the toe box. I am curious what the next step should be as the store owner only carries D widths? Should I try for an 8.5D or try 9C? I am worried that the 9C would be too tight around the toe box.

I will also add that I am flat footed, not sure if that factors into the equation, thanks!