Amazon sells those as "bag cutters" and I wouldn’t be without one.
https://www.amazon.com/gp/product/B015X6CJX6
My main use is (gasp!) cutting vacuum bags, but it works pretty well for opening padded mailing envelopes, cutting clear film, etc. Really a very cost-effective tool. It doesn't get very dirty but if it encounters some liquid I'll rinse it or give it a ride in the dishwasher. The hole in the end slips nicely over one of the divider wires and keeps the thing in place.
Edit: I just realized that Amazon is asking six bucks apiece for these/pkg of two. I'm sure that's twice what I paid. camelcamelcamel.com shows a lot of price gyrations. If you're a cheapskate like me you may want to wait until the price comes back down.