The best sources for fruit, nut and berry stock are local nurseries and gardeners that produce their own stock and have experience growing the varieties that they offer in our climate, so seek out your local nurseries!
You have the choice of buying fruit, nut and berry plants as bare root or container stock, each have their specific advantages and requirements for handling and planting. If grown and handled properly each will perform equally well in the garden/ orchard.
Bare root stock - Plants that go through a dormant period can be dug up when dormant and sold with no soil around the roots, thus "bare root". This includes most temperate zone fruits, nuts and berries that lose their leaves in the winter. The advantage to bare root plants is that you are paying only for the plant and not the cost or weight of the soil and container required to grow and transport it, the disadvantage of bare root stock is that you are restricted to buying and planting during the winter. Evergreen plants (such as citrus, pineapple guava) cannot be bare rooted, and some plants do not respond well to the shock of having all of the soil stripped from their roots (kiwis, for example). Bare root stock is available through local nurseries and also mail order. Ideally it's best to put your order together early in the season (November or December if possible) and plant early (January or February) to ensure the best selection and to avoid ending up with stock that has been stored too long, resulting in desiccation or breaking of dormancy, though you can get away with planting bare root stock into April.
Container Stock is just that, plants grown and sold in a pot with soil. The advantage of container stock is that you can purchase and plant your stock year round and container stock can be transplanted with a minimum of shock (though it is best not to plant stock in the heat of the summer. The trade off is that you pay for the container and the soil, and thus may end up with a smaller plant than bare root for the same price, however, at the end of the first season potted stock is typically the same size as bare root stock. When buying container stock make sure that the plants have not been left in their container too long - plants should reveal uninterrupted growth and should not be root bound. If you are buying branched fruit trees be sure to select well-angled crotches that will grow into branches that support the weight of a full crop. Narrow crotch angles are weaker and result in branches that re likely to break under a load of fruit. Use the "rule of finger" - when you place your finger in a branch crotch you should not be able to see light through the crotch, if you can then it is too narrow and likely to break under a heavy crop.