If you are only looking to pipe the same source to one or more of the zones, all you need is an amplifier, source(s) and a speaker selector switch.
The amp doesn't have to have a crazy amount of power, 100W is plenty and you can probably even get away with less.
The source(s) can be pretty much anything that can connect to the amplifier.
Finally you will need a speaker selector switch like this:
http://www.monoprice.com/Product/?c_id=109&cp_id=10903&cs_id=1090305&p_id=8229&seq=1&format=1#largeimage
Just connect the speaker output of the amplifier to the input of the selector switch and connect each volume control to one of the outputs on the selector switch.
Now, there are more complicated and expensive solutions that will let you play different sources in each room, but without control wiring to each zone, control can get complicated.
Hope this helps?
-Mario