If you have a script then you can legally posses the amount on the script so where you order it will likely depend on the rules of the pharmacy. There are a lot of big time pharms doing mail order so I dont see what you couldnt mail order it if you think its cheaper. Mail order will probably want a lot more personal info from you and probably call your doc to verify script. Im sure people try and pass off fake scripts to them all the time. You may find its a lot easier to just get it at a local pharm.