You're going to have to get some more specifics from your airline. Even with that, though, sometimes you get screwed and have the seat with a little less clearance than they publish.
Personal items typically fit under the seat in front of you, so you can look for "underseat carry on bag" to get a rough idea of what is available.
Bulkhead seats usually have reserved overhead storage. The attendants will often offer to stow your bags for you right before takeoff, so that you can access your bags right up until they need to be stowed.
It's not exactly those dimensions, but check out Osprey Daylite Tote Pack. I read some airline summary and the OP said it fits every single one, even the more restrictive ones
I have a North face shuttle daypack that is exactly the nominal dimensions and conveniently boxy. I'm 90% sure it's fake though, so going with a ruler to a store or market that sells backpacks is what I'd recommend.
There is a company making a hard sided suitcase with removable wheels which can fit. I'd strongly recommend against that because the handle will take up so much of the already limited space. And there's nowhere to put the wheels when they're removed where it can still fit. Note that Spirit sizers are unofficially 18x14x9 rather than the officially advertised 18x14x8. Frontier's bag sizers are truly 18x14x8.