I had forgotten about slingbacks! It's been a little while (thankfully!) since I had that job. Also as I've been thinking, I do remember hearing about narrow heeled/medium width shoes. I think the brand may have been Naturalizer? You might call shoestores in your area and ask about combination widths.
Also, we used to stretch leather shoes successfully for customers, but it actually works even better at home. If you find a shoe that fits your heel and is only a little too tight across your width, you might take it home and stretch it. Mix equal parts water and rubbing alcohol and get the inside of the shoe wet where you wish it to stretch. A squirt bottle works great. Applying the mix on the inside minimizes the chance that the dye in the leather will be affected. Wear the shoes until the mix dries, and the leather will be more form-fitted. Sometimes the leather will stretch well, but then tighten back up in the night. If that happens, then just apply the stretching mix again and wear the shoes again. In my experience, shoes that were resistant to stretching would finally relax into a stretch after several applications.
This only works for genuine leather.
People used to come in for us to stretch their shoes for them, but our commerical stretching spray was just water and rubbing alcohol, and although we had wooden foot forms, they obviously were general foot shapes and couldn't imitate the nuances of a customer's actual foot.