Your local home improvement store will surely have a "fire safety" section. This will be the best place to buy, because the items are bulky and stored under pressure. This makes them expensive to ship.
If you currently have no fire extinguishers, I would suggest at least 1 per floor. Keep one under the sink in the kitchen, and store the others mounted to the walls in your closets.
You are looking for an "ABC" or "tri-class" dry chemical extinguisher. These will generally come in a wide variety of sizes. Get a moderately sized one for your kitchen, and the same size or smaller for the other areas of your house.
In case you are curious, the "ABC" rating refers to the class of fires which the extinguisher will be effective on:
- A denotes a fire in combustible materials (wood, paper, etc.)
- B denotes a fire in combustible liquids (oil, gasoline, etc.)
- C denotes an electrically energized fire. The rating ensures that the chemical in the extinguisher is non-conductive.
- D denotes a fire in flammable metals. You are not likely to see this type of fire in a residential setting.
There will also a numerical rating for the A and B classes, which denote the relative extinguisher strength. The A number multiplied by 1.25 will give you the number of gallons of water that the extinguisher is equivalent to. The B number will give you the approximate number of square feet of B class fire that a person with no training can expect to extinguish.