Car booster amps can produce quite a lot of power - I use one for some small open air busking-style venues and it's not bad - but to get the most out of them use 4 ohm speakers.

The easiest way is to use your mains PA running from a car battery via an inverter. These are getting very cheap these days - I bought a 300w one for about £30 recently. Obviously if you were running it at a steady 300w, you'd be dragging in the region of 30a from the battery. In fact the average power required by an amplifier is usually only a fraction of it's rated power which is only reached for momentary peaks. These peaks are averaged out by the large smoothing capacitors in the power supply, so a 300w inverter can supply an amplifier with quite a higher peak output.

You just attach the inverter leads to a car battery and plug into the mains socket.