A spore print is a piece of paper (tinfoil or glass slide) used to collect the spores of a particular strain.
To start growing mushrooms one must first get a spore print. A spore print can be obtained from a mushroom found in nature.
Spore prints are also widely available on the internet. Several websites dedicated to the cultivation of mushrooms offer spore prints for sale. There are also numerous forums where cultivators exchange spore prints. In most countries it is not illegal to possess a spore print.