The national chains seem to be shying away from selling refills (or sometimes they are called “inserts”). I asked an employee at one of these stores, and he said the reason was that DIY customers would buy the inserts but not be able to figure out how to install them correctly, and a big scratch on the window would result. So he said refills became a liability problem for the store.
Not all auto parts stores refuse to sell refills. My local (non-chain) auto parts store offers them for example. The problem you may have is getting a refill that fits the wiper blade assy you already have. If that’s the case, your best bet is to purchase new wiper blade assy’s this time, but make sure the store stocks refills that fits that part. Trico, as mentioned above, is a brand that seems to be happy to offer compatible blade/refill pairs. Maybe buy a couple sets of refills at the same time as you buy the blade assy’s, then you’ll be certain of the refill part number when you need it.
Coincidentally, I had this problem for my 90’s Corolla recently. Refills – I discovered – consist of just three parts, the rubber part, and two metal wires which provide flexibility and mechanic support. I wasn’t able to find refills which fit my existing blade. But I found refills where the rubber part was the same. But the wire part was a different width. So I purchased the refill, used the wire part from the existing blade, with the new rubber part from the refill, and that worked out fine. And saved me quite a few dollars compared to purchasing new blade ass’ys.
The other thing I should mention since I have recent DIY’er experience w/this topic is that I discovered almost by accident that I could purchase the entire blade ass’y at some web parts stores for about the same price of an insert at a local parts store. So it might be worth taking some time to see what some of the web parts stores offer too. Best of luck.