how does one go about getting one?
can it be gotten online before arriving?
This is not an exact answer to your question ("monthly") but those passes are explained on Page 34 of RS Venice 2012 Guide Book. It shows passes for
7 days and yearly. Good luck and I wish I was going to be able to stay in Venice for a month!
From what I see, there is such a card. It's called the imob.venezia card. Apparently, you can buy this card at the Hello Venezia ticket booths. The older card used to be for residents only but this card seems to be for both residients and visitors. Here's a LINK to information I found.
The official site of the vap. system is www.actv.it and it has an English version. You may also want to check a site www.veniceforvisitors.com which has a world of information. TC
Yes, the www.veniceforvisitors.com website gives a step by step procedure to get an imob card. Since it is a photo ID card, you need to pick it up in person. It apparently costs 40eu and is good for 5 years, then you pay the locals rate (1.30eu?) per ride rather than the "tourist" rate (7.00 eu?)