I found it in the GECK although it seems you're outta luck. As far as I can tell it's not used by any NPC or is placed anywhere in the world.
So not even a random NPC can spawn with it? Looks like it's lost content, like the "Papa Khan Helmet," "Legate's Armor" and "Jason Bright's Outfit"
I think if you win over a certain ammount in the atomic wrangler casino in Freeside, you get a similar one.
It may not be the exact hat your looking for, but a daper gambling hat is one of the gifts for winning there.
Have you tried killing every one in the strip a few times? i.e go away wait a week then return.
Trust Me, I killed EVERYONE, even people not wearing any hat, because sometimes they don't wear one, but they have one in their inventory. Not one person had any. I found tons of "Dapper Gambler Hats," "Fancy Gambler Hats," and "Stylish Gambler Hats," (And already had the "Suave Gambler Hat") but did not find a single "Well-Heeled Gambler Hat". Another problem was that no one respawned no matter how long I waited, so that plan was full of holes.
It's near Goodsprings in a fridge beside the road that leads to to Jean Skydiving.
That's the "Suave Gambler Hat". It is NOT the hat in question. You should try to read more carefully.