Really, what you want is a boombox, ghetto blaster, or equivalent. Speakers, CD player, tape deck, all in one unit. Run on mains or (lots of) batteries (6-8 LR20s, usually). Fundamentally, you get what you pay for - A cheaper model will inevitably be less powerful, and as you get more wattage, the size (and bulk) increases too.

Another option might be portable speakers, into which you could plug a portable CD player or mp3 player (not an iPod dock, just speakers that you can connect to the headphone socket of the portable CD). These will tend towards being cheaper, but of course, correspondingly less powerful.

The final option is to prevail upon some electrical engineers, and get them to build you a rig out of parts. The most impressive one I've seen recently was two 100W Wharfedales, connected to a mixer, powered by a car battery, mounted on half a luggage trolley. Obviously, yours wouldn't have to be that potent, but the principle remains - You could, for example, have a smaller setup that fitted onto the frame of a senior citizen shopping trolley.

Summary: There' lots of options, the hard part is figuring out how loud you'll need.

the_magician has LOTS of PA and stands which he really ought to dispose of before his move, so have a chat with him.