Get an old home theater receiver...they have as many as 7 outputs for speakers and they have plenty of inputs for whatever you want to listen to. On stereo inputs, they just 'make up' the rest of the channels and put sound through the rest of them. The bonus is that you can put a subwoofer somewhere in the garage (hang it from chains in one of the corners).
Brands which have worked well for me and relatively inexpensive: Yamaha, JVC, Onkyo. No need to go for new, look for used units on CL. I have an old JVC unit, which I replaced with a Denon, that I plan to use in the garage.
Speakers are the same thing...just find some decent, used speakers on CL. If you don't mind doing a little more work, get the new, inexpensive speakers and crossovers from a provider like Parts Express, and build your own MDF cabinets. The other option is a 6x9 full-range car speaker...they are built to survive brutal environments and still sound OK (your car).
Depending on the size of your space, you could also get away with a set of the computer surround sound speakers....they usually have all the amplifiers built into the subwoofer cabinet and you connect the satellite speakers to plugs in the woofer. Of course, you are going to need to extend those cables (use lamp cord from the big box stores)
Kev