(1) you could just wait. Infuriating, I know, but if you can manage your life without access to that money for another couple of weeks or so, that might be the easiest and will almost certainly be the cheapest way to go.
(2) I guess you could try writing them a letter in which you remind them that Montana law (Montana Code Annotated section 70-24-109) requires them to act in good faith, and that their failure to promptly return an erroneous rent payment looks like bad faith to you.
(3) you could go ahead and sue in small claims court. I'm pretty sure you don't need a lawyer to sue in MT small claims court, but if you do represent yourself, that means you have to show up in court (unless the defendant chickens out and returns your money before your court date). So if you moved far away and have a busy life, that could be pretty inconvenient and might even cost as much as hiring a lawyer. I read online that the judge has the discretion to make the defendant pay your legal fees, but might not do so, so if you hire a lawyer you risk being out of pocket what you pay him or her.