Tom,
I personally have had a lot of success with Banner's QS18VN6D (IR) and QS18VN6LV (Visible Light - Red) sensors for these types of FRC applications.
They are close to a drop-in replacement for the ABs, but these only need 12V.
Sourcing them for us has been easy and fast because they used to come in the KoP, and being an older team we've built up a stockpile of them. You could probably call up one of more generous very old MI teams, and there's a good chance they'll have a bunch kicking around.
Else, you can order them direct from Banner at
http://www.bannerengineering.com
We're using the QS18VN6D (IR) right now on our 2013 robot.
Sometimes it's easier to prototype with the QS18VN6LV (Visible Light - Red) since you can see the light being emitted, making alignment much easier. Even though the specs say it is supposed to be used w/ retro reflective tape, at close ranges (i.e. < 1ft) a black/white painted/coloured regular diffuse surface will work fine.