While you really should do this in person, I had one Prof. that was REALLY busy (I think he was up for tenure.) When I tried to make an appointment to see him (he wasn't teaching classes that semester and didn't have any office hours,) he pretty much wouldn't agree to see me until I told him what it was about. I eventually asked him in email, and he said something like "Sure, I'd be happy to -- there's no need for you to stop by, just send me PDFs of your CV, etc."
I would highly recommend asking in person, however (which I did for all of my other LORs -- except for a former boss that now lives in India.)