What Macyver said.
Your best best to my personal knowledge is Los Algodones, Baja California. About 10 minutes west of Yuma, AZ on I-10, and 1 mile south of same. Park in the Indian lot for $8/day and walk across. Once in the morning and once again in the afternoon will net you an allowed 3 months supply each time across. We have been to Mexicali and Nogales as well, and they can't hold a candle for safety, decent vendors, and friendliness.
I would suggest getting in touch with places like Phoenix and Purple pharmacies to see if they have your meds. If not they might be willing to order them. Medical tourism is the lifeblood of that community and they safeguard it well.
The only things you will have issue with are controlled substances - even Tramadol now - and Cipro, and injectables in general. This with Customs, not the pharmacies. Why Cipro is anyone's guess but the 'ban' has been off/on for a dozen years.
Do be aware that during snowbird season the return line can easily be an hour and a half long, and the car lane as bad. Bringing back sacks of meds is better if you walk; beats getting directed to the side for a thorough going through. Carrying a small daypack that includes a few tourist items as well helps. I usually carry spare boots down for the shoeshine guy, in addition to what I am wearing. No one has ever asked me to show them the toes weren't packed (they weren't) nor even allowed me to empty out the pack entirely. Carry no more than 90 days supply per person, in the original pharmacy bags, and keep the receipt showing you only bought that amount, even if it means having them ring things up twice for your spouse or other companion.
The Indian casino at the border is nice. It is very convenient for 2-3 times a day trips. Otherwise a decent place in Yuma if you decide to drive once, park once, and make 2-3 trips. Do a morning run, go back and have lunch, then an afternoon run. You can make a 3rd run if you go back to buy again and then have supper, and come back for the final time for the day.
RR