First you need to figure out the wattage you will need.
Then buy the appropriate inverter. Inverters are getting cheaper ever day. I use a 220 watt inverter for power outages at the home ($40-50) and it will run my 20" TV and a 40 watt light bulb with no problem. I use a 600 watt inverter ($60-70) in my truck for builders who don't believe houses need electric power to be built and hence don't provide a power source. This will power most of my power tools. A 2500 watt (about $500) will power just about anything with a plug.
Depending on the kind you get, they either plug into your cigarette lighter or are attached to the terminals of your vehicle battery. I would recommend, however, that you purchase a seperate battery, marine perferably.
Inverters can be found at most auto part and marine stores.