I believe we are talking "POT" here an in some states it's still illegal even with a prescription but not all ... So sending such thought the mail is against Federal law I believe an no Walmart or Walgreen's will carry such an it only will be available in a certain type of shop which will not be available in every state
Unless the OP can get a 90 day prescription for such he is more or less stuck to staying in the state were he can obtain such, but as stated the doctor has limited him to a 30 day supply so he is sort of SOL in many ways till the government makes it not illegal across the board to obtain such ...
Good luck Safe travels