My 72 year old Mother has been taking 5mgs of Xanax a day for several years and the prescribing doctor is retiring. Any suggestions?

This is for her present doctor to address. You would get your mother registered with an MD at once. You would attend with her, and a list of her medications. If your mother is to be withdrawn from this drug by another doctor it must be done CAREFULLY titrating it down so as to avoid severe withdrawal. You should arrange to have Durable Power of Attorney for health care in case needed, to assist your mother. It is absolutely crucial she not run out of this medication, even if that necessitates a visit to an Emergency Room; they will there assist you in seeking MD followup suggestions for your Mom. Good luck.

Is your concern that she is either taking too much/she's addicted, or is your concern finding her a new doctor?
If it's a matter of finding a new doctor for her, you can ask for guidance with her current doctor, such as what questions to ask, if she needs a specialist, etc.
If your concern is she's addicted, and you don't know who to call, if it were me I would start with my local county health office for guidance. You might also want to contact your local chapter of NA and ask for their guidance, those two organizations should be able to steer you in the right direction.
Good luck!