Your primary doctor or insurance company website should be able to refer you to pain management.
Did you get your records from your previous doctors office? If not, you are going to need to get them somehow from whomever bought the practice.
The problem you likely will run into, assuming there were no violations of your contract with your previous doctor, failed urine tests, etc, is your going to a methadone clinic, rather than seeking treatment with another PM office after your doctor passed.
Methadone clinics do not treat pain, and your attending programs and going to obtain methadone there all this time is likely to be viewed as an addiction issue rather than a pain issue.
So it's very important you get your prescription fill history, your records from previous pain management, tests and results, and any other documentation you have regarding your condition and previous treatment.