Customization items aren't hard to come by, there are heads, and skins (vehicle or character).
There are over 50 skins to get from completing missions and challenges, defeating enemies, the Item of The Day vending machines, slot machines or just drops from enemies/bosses.
If you are having trouble finding customization items here are some challenges/missions that will get you them. When you get them you have to go to your backpack and USE THEM, do not drop them, they'll automatically go to the change station/Pick-A-Ride area when used.
These can be found across multiple playthroughs, so you can find 4 on one character and 6 on the other. Thanks to Salvatore 2 for the find.
Heads (Most are found by drops)
Defeat any badass.
Defeat hidden enemy in Caustic Caverns...
Defeat first target in "The Talon of God"
Defeat second target in "The Talon of God"
Defeat target of
"You.Will.Die. (Seriously)"Defeat boss of mission "Best Mothers Day Ever"
Defeat boss of mission "Where Angels Fear To Tread"
Complete mission "Symbiosis"
Complete mission "In Memoriam"
Complete mission "Statuesque"
Complete "...To Pay The Bills" challenge (Kill 1000 enemies with action skill)
Skins (These are all missions)
This Town Ain't Big Enough
No Vacancy
The Cold Shoulder
The Overlooked: Medicine Man
Capture The Flags
To Grandmothers House We Go
Cordially Invited: Party Prep
Shielded Favors
33 are earned by challenges
8 are from Item of The Day
2 are rare prizes in slot machines
14 are drops from enemies and bosses
8 are mission rewards
If you are wondering where to find these missions just comment and I'll add them onto it.