How to Buy a Petri Dish
The most obvious place to buy a petri dish is from a science supply store but, depending on what you are using it for, there are cheaper alternatives. If you are using your petri dish to examine plants or small animals, you can find cheap plastic containers of the same size for much less. If you want to use the petri dish to culture bacteria, mold, or yeast, however, you should spend a bit extra to get a sterile petri dish to start with.
Instructions
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Think about what you are using your petri dish for. Are you going to be using it to observe an animal, plant or rock, to culture microorganisms, or for some other purpose? Does it have to be completely transparent, or can it be semi-translucent? Does it have to be sterile? Reusable?
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Consider buying small food-service condiment cups such as the one listed below. These are about the size of petri dishes, and are useful for looking at critters with the naked eye or under a microscope. Best of all, they are very cheap.
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Look at dollar stores and thrift stores. Although you are unlikely to find a petri dish per se, you may be able to find a small, inexpensive dish useful for your purposes.
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If you need a sterile dish, order it from a science supply catalog, such as the one linked to below. Plastic dishes are cheaper, but glass dishes can be cleaned in an autoclave and reused.
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If you want to grow micro-organisms, get agar to grow them on. You can order premade agar plates, or petri dishes with agar which you can pour in when you are ready to use them. There are a variety of different types of agar used to grow different sorts of microscopic critters. Nutrient agar plates, such as those listed below, are good general-purpose petri dishes for basic science experiments.
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Tips & Warnings
Although an autoclave is necessarily to completely sterilize a petri dish, for some sorts of experiments, hot water and soap or rubbing alcohol will do. If you just want to observe the little critters swimming around in pond water, for example, you should have a clean dish, but you won't need a sterile one.
Be careful to not inadvertently contaminate your agar plates or petri dishes. Even a small amount of contamination can grow whole colonies of bacteria of fungus.
