It took a long time for the finale to show up for me - I had set off one of each type (including twisted), 75 sober firework encounter, 20 firework serpent fights, and then used two twisted ones, fought an intertwined serpent and then got the finale very next time.
You find a street vendor picking up his blanket and wares. "Hey, saw you enjoying the fireworks on Metroplex Day. Good show, eh?"
"Here, I've got some extra merchandise from then. Can't move it any other day, so you can have it."
He slips you a handful of merchandise and heads on his way.
You found: saturated firework
You found: eternal sparkler
You found: fireworks shades
You found: 15 charred fireworks