The reed switch is probably the most critical piece of this project. I actually had a hard time finding these little jewels. The ones I found were at frys.com and were called Philmore Open Reed Switch. I think the actual part number I used was Philmore 30-17154. There is virtually no current that flows through them (0.5 microamps) so there is nothing to worry about regarding current rating.
This switch has connections for normally open (NO) or normally closed (NC). Only the NO lead is needed so I cut off the NC lead and just used the NO lead. I used an ohmmeter to figure out which lead needed to be cut off (it was the longer of the two leads on the one side), the lead that needs to be used should show as an "open" when no magnet is used and "closed" when a magnet is used.
There is really no reason to get this same switch, a standard normally open reed switch (with only two leads instead of three) will work just fine. I found a Philmore 30-17152 that is much cheaper and only has the connections you need.
Here are some links on the switches I found.