...because you need a bonsai
Sources of Bonsai Material

All bonsai start with a specimen of source material, a plant that the grower wishes to train into bonsai form. Bonsai practice is an unusual form of plant cultivation in that growth from seeds is rarely used to obtain source material. To display the characteristic aged appearance of a bonsai within a reasonable time, the source plant is often partially-grown or mature stock. A specimen may be selected specifically for bonsai aesthetic characteristics it already possesses, such as great natural age for a specimen collected in the wild, or a tapered, scar-free trunk from a nursery specimen. Alternatively, it may be selected for non-aesthetic reasons, such as known hardiness for the grower's local climate or low cost (in the case of collected materials).
Propagation
While any form of plant propagation could generate bonsai material, a few techniques are favored because they can quickly produce a relatively mature trunk with well-placed branches.
Cuttings. In taking a cutting, part of a growing plant is cut off and placed in a growing medium to develop roots. If the part that is cut off is fairly thick, like a mature branch, it can be grown into an aged-looking bonsai more quickly than can a seed. Unfortunately, thinner and younger cuttings tend to strike roots more easily than thicker or more mature ones. In bonsai propagation, cuttings usually provide source material to be grown for some time before training.
Layering. Layering is a technique in which rooting is encouraged from part of a plant, usually a branch, while it is still attached to the parent plant. After rooting, the branch is removed from the parent and grown as an independent entity. For bonsai, both ground layering and air layering can create a potential bonsai, by transforming a mature branch into the trunk of a new tree. The point at which rooting is encouraged can be close to the location of side branches, so the resulting rooted tree can immediately have a thick trunk and low branches, characteristics that complement bonsai aesthetics.
Commercial Bonsai Growers
Commercial bonsai growers may use any of the other means of obtaining starter bonsai material, from seed propagation to collecting expeditions, but they generally sell mature specimens that display bonsai aesthetic qualities already. The grower trains the source specimens to a greater or lesser extent before sale, and the trees may be ready for display as soon as they are bought. Those who purchase commercially-grown bonsai face some challenges, however, particularly of buying from another country. If the purchaser's local climate does not closely match the climate in which the bonsai was created, the plant will have difficulties surviving and thriving. As well, importing living plant material from a foreign source is often closely controlled by customs regulations and may require a license or other special customs arrangement on the buyer's part. If a local commercial bonsai grower does not exist, buying from a distant one may be unsatisfactory.
Nursery Stock
A plant nursery is an agricultural operation where (non-bonsai) plants are propagated and grown to usable size. Nursery stock may be available directly from the nursery, or may be sold in a garden centre or similar resale establishment. Nursery stock is usually young but fully viable, and is often potted with sufficient soil to allow plants to survive a season or two before being transplanted into a more permanent location. Because the nursery tree is already pot-conditioned, it can be worked on as a bonsai immediately. The large number of plants that can be viewed in a single visit to a nursery or garden centre allows the buyer to identify plants with better-than-average bonsai characteristics. According to Peter Adams, a nursery visit "offers the opportunity to choose an instant trunk". One issue with nursery stock is that many specimens are shaped into popular forms, such as the standard or half-standard forms, with several feet of clear trunk rising from the roots. Without branches low on the trunk, it is difficult for a source specimen to be trained as bonsai.
Collecting
Collecting bonsai is the process of finding suitable bonsai material in situ, successfully moving it, and replanting it in a container for development as bonsai. Collecting may involve wild materials collected from naturally treed areas, or cultivated specimens found growing in yards and gardens. Mature landscape plants which are being discarded from a building site can provide excellent material for bonsai. Some regions have plant material that is known for its suitability in form. In North America, for example, the California Juniper and Sierra Juniper found in the Sierra Mountains, the Ponderosa pine found in the Rocky Mountains, and the Bald Cypress found in the swamps of the Everglades.
The benefit of collecting bonsai specimens is that the collected materials can be mature, and will display the natural marks and forms of age, which makes them more suitable for bonsai development than the young plants obtained through nurseries. Some of the difficulties of collecting include getting permission to remove the specimens, and the challenges of keeping a mature tree alive while transplanting it to a bonsai pot.