I have a rapport with one of my professors from undergrad 10 years ago. I just sent him an email asking if I could meet with him for a chit chat. We talked about life and research and where the years have gone until the discussion turned towards the real intent of the visit at which point he told me he would gladly write me a letter.
The prehealth advisor from my old school suggested a letter from some people at my graduate institution, a letter from my supervisor, and a letter from people I interact with professionally on a daily basis.
oddly enough the letters from my graduate institution are becoming the most difficult- they won’t respond to their emails.