I really feel for you. I've heard from so many people who have taken Darvon or Darvocet for years and are in a similar situation.
I don't really know what countries might still be using Darvocet. I do know it was discontinued in Canada and throughout Europe. The problem is, even if you manage to find a country that still allows Darvocet to be prescribed, it is illegal to import it or any other opioid (narcotic) drug into the U.S. It is also illegal in most countries of the world to export opioid drugs. So if you find a pharmacy that says it will fill your prescription, it is operating illegally. Most, if not all, of the online pharmacies who do this are scams. They are set up to get your credit card information; or they take your money but don't send you the medication; or perhaps worse, they send you something, but there's no way of knowing what ingredients are actually in the medication you receive.
Please read this article before you consider ordering medication online: A Guide to Buying Prescription Drugs on the Internet
I know this isn't the answer you hoped to get. I'm just afraid if you try to order Darvocet from another country, you could find youself in a lot of trouble.
Is Ultracet one of the medications you've tried? I just heard from another former Darvocet user who said they tried Ultram (tramadol) and it didn't help, but then they tried Ultracet (tramadol plus acetaminophen) and it did a pretty good job.
I don't know what kind of pain you're dealing with, but you might also want to look into some other treatment options like a TENS unit, acupuncture or a topical pain reliever. I do hope you find something that helps you very soon.