The IXEG 737-300 is but works in XP12 with some caveats:
Inside each livery folder, look for: objects/ (contains fuselage_t.psd/png, wings, engines, etc.) livery.json (optional – for icon/description) ixeg 737300 liveries
A true throwback to the late 80s and early 90s. How to Install IXEG 737-300 Liveries The IXEG 737-300 is but works in XP12