They are hard to find now, but before this virus they were everywhere. Many people I know, including myself, have always had N95 masks on hand. If you work with or around wood, drywall, spraypaint, fiberglass insulation, do any sanding or home improvement/hobby work, these things are standard equipment. They are used extensively in non-medical industry and trades. Most people probably got them from Home Depot for use at home or from someone they know in a trade.
There are two major hospitals in this area. One of them is tier 1 research hospital and major academic hospital.
Healthcare is a very significant part of the local economy.
That would be my guess as to where most of them got the medical masks.