Is there medicare PTAN or Provider Transaction Access Number look up website?

You can check the NPPES NPI national Plan & Provider Enumeration System and if the Provider has kept their records up to date, the information could possibly be in that system. Otherwise, it is the responsibility of the providers office to keep accurate records of their PTA numbers and there is no Medicare website to look up your lost number. You can find the number by looking on an old Medicare remittance that your office has submitted and it should be clearly identified on the document.