Incubator is a warm cabinet that you can set its temperature to a proper temperature for bacteria growth. About 35º C is a good temperature for most bacteria. This is close to the body temperature. If you are able to create such a temperature in any other way, it is as good as an incubator. You may find warm places behind your refrigerator, next to the radiator or inside an oven that is off.
You may also make an incubator by placing a small desk lamp inside a wooden or metal box. Or you may put a Styrofoam cooler upside down over a desk lamp. A small lamp (15 watts) should be able to create enough heat to warm up a small space. Prepare your incubator in advance and use a thermometer to test it a day before starting your experiment.