While it's possible to buy mushroom spores, these spores are actually so tiny that they're microscopic. This means that they can be quite difficult to handle, at least until you get used to things. You can also harvest spores from mature mushrooms, simply by cutting the cap off and placing it on a large sheet of paper or on a sheet of glass. However, I would advise against using spores, not only because they are inconvenient to handle (a single breath of wind will scatter them all over the house or yard), but also because they're vulnerable to contamination, and if they contaminated with spores from wild, poisonous mushrooms, the consequences could be disastrous.

Preventing contamination is also the reason why it is recommended that you always go in for mushroom growing indoors. Indoor growing greatly reduces the chances that a few wild spores might come to rest in your mushroom beds, and grow up among your safe mushrooms. If the mushrooms that grow from the wild spores turn out to be poisonous, it could cause serious problems for anyone who eats them. And that's just one reason why you shouldn't grow mushrooms outdoors.