Those are called COVER (SEAT SHOULDER STRAP RETRACTOR) and you can find the illustrations and part numbers here: Interior/
Interior Trim. Go to the Illustrations tab and then Bronco tab and scroll all the way to the bottom for the drawing. And the part numbers are on the Part Numbers tab, oddly enough.
(That's the only part numbers, and they just appeared tonight.)
I can't tell you where to find them, but once you have the part number you can search for them. And, since you can paint them you don't have to find them in your color.
Gary, AKA "Gary fellow":
Profile
Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI