You can go to Amazon.com and look up “Frontier, Spirit Airline bag.”
Well, spit. That didn't work. Seems anyone who has a travel bag smaller than 18x14x8 is marketing it as a "Frontier/Spirit bag", whether it was designed to fit the personal size regulation or not. Case in point is the Wandf bag that's touted as a Frontier/Spirit bag. It's only 70% of regulation size (about 1400 in³).
At least the F.Fetivin bag looks like it was actually designed to meet the Spirit/Frontier size, but it doesn't open fully or have backpack straps.
The Sucipi bag opens fully, but it, too, lacks backpack straps.
The RS appenzell works as an underseat bag.
As I said above, I have an Appenzell bags, which I don't use 1) is just barely fits all the stuff I take to Europe and 2) I don't like the way it loads.
I have an old Travelwell bag, that isn't sold anymore. It's kind of a junior convertible backpack, with straps, and it opens flat. It's also only 1400 in³. I have used it for short trips. It would fit the personal item regulation.
Fortunately, I'm not planning to fly Frontier or Spirit, so I'm not restricted to 18x14x8.. I would just like to find something smaller than my current eTech convertible Jr. at 2350 in³.
I have a Hynes Eagle. It's advertised at 2023 in³; I think it's considerable smaller, but it might work.
I see that Wandf also has 30 & 40L bags. Their weights are a real attraction, but again, no backpack straps.
Still, I would like to see our light packing guru sell a bag for that purpose.