Are you in California? Then, the answer would be easy to get: Walk into a gunshop and try to buy one.
If you're not in California, just 'phone any randomly-chosen gunshop in California, and ask if they sell .45 ACP Px4s.
You could call Martin B. Retting, in Culver City, California, at 310-837-2412. Their e-mail address is firstname.lastname@example.org.