There are several options if you don't limit yourself to those sheets still in production. OOP decals can be found on eBay, model meets and at some of the better hobby shops. Aeromaster and Superscale(Microscale) produced several sheets of 56th FG markings with Gabby's bubbletop included. There's an older sheet from HisAirDec (dry transfer) that turns up from time to time. The older Hasegawa D-Day commemerative Bubbletop in 1/72 has Gabreski's markings. Techmod produces a 1/72 scale as well.
There's also an older sheet from AIR., and one more that I just don't recall at this time. In quarter inch there's much more variety, but that's another story!
"There's no problem that can't be solved with a suitable application of high explosives"