Where to find suitable nursery stock
Buying a prebonsai is an rather inexpensive and fast way to increase your Bonsai collection. One major drawback is that starting work on an older tree (instead of growing it right from the start) often means you will have to remove large branches, with visible wounds as a result.
Good quality nursery stock can be hard to find, try looking at small family owned nurseries (large commercial stores often don't sell rough material). Some (online) bonsai shops sell nursery stock as well, but this comes at a price higher than at small nurseries. Check this article for a store near you; Bonsai trees for sale.
Nursery stock can be bought throughout the year, but most material is offered from early spring till late summer. The best time to actually work on a prebonsai is in spring, or early fall.
What to look for in nursery stock?
This is rather hard to explain, your skills to find potential material will improve once you learn more about growing Bonsai. However, key aspects to look for are the tree species, the shape and tapering of the trunk, the tree’s branching structure and its Nebari (root flare or surface roots). The photos below show examples of good quality prebonsai, very suitable to be trained as a Bonsai.
Young prebonsai material (Chamaecyparis), around $20.
Young Picea nursery stock, around $20.
Styling nursery stock material
Place the newly bought nursery stock outside (unless it is an indoor tree obviously) and keep it damp, but not wet. The trees can be fertilized using normal quantities as described in the fertilizing section. It depends on the season in which you buy the nursery stock whether or not you can start styling the tree right away. When bought during the early spring you can prune or repot trees immediately, but if you buy trees later in the year you should wait until early fall, or the next spring.
Only work with healthy material that shows signs of strong growth. To ensure the health of your tree, do not repot, prune and wire all at the same time. Ideally you repot the tree in early spring, and wait with any structural pruning until late summer.
For reference material about training nursery stock check the Progressions part of this website. Or read the Bonsai styling articles for background information about techniques like pruning, wiring etc. An excellent way to get a taste of this hobby, is to buy a Bonsai tree DIY kit, and grow a Bonsai yourself!