It's long gone. PPG discontinued it back in 2003 or 2004. Going to be real hard to find any NOS. Epoxy or urethane would be far more durable. SPI 2201 can be mixed in a variety of ways to achieve whatever gloss level you desire. Although the Tech Manual doesn't specifically mention it you could mix the 2201 5:1 and that would be very close to 9423. I have done that recently on the underhood/engine bay of a 69 Mach 1 and it is spot on. Owner was very pleased. You can mix it anywhere from 4:1 up to 8:1 depending on the gloss level desired. Plus it is a urethane which will be far more durable than lacquer.
I'll check with my jobber tomorrow, they are a Platinum PPG dealer so they are supposed to have the entire line. But I know that I stopped being able to buy Duracryl back in 2004 around here, I did find a TDS on the PPG website so maybe they have brought it back in a reformulated version. If you have a PPG Platinum jobber in your area check with them. If they say they don't have it or it's not available then the chances of you finding any will be slim to none.