There are two main choices when it comes to buying plants; specialist nurseries or garden centres.
Specialist nurseries are the backbone of the horticultural establishment and since many of them struggle against the odds, they deserve as much support as they can get from the plant buyers. There are many advantages to buying plants from a specialist nursery, starting with the sheer range of plants on offer. Many nurseries specialize in cultivating a single group of plants, selling just palms or bamboos or others. Such nurseries often have a wide range of sizes available. In addition nurseries are usually fascinating places to visit in their own right.
One advantage of buying plants from garden centres is convenience - they are usually located in town and are easy to reach. Garden centres often stock huge range of sundries such as potting mix, mulch, garden tools, pots, trellis, plant food, hose pipes and just about everything else you would need for your gardening. Hence plants may not always be at the top of their agenda, but if you can find one that sells all these sundries and even a few of the architectural plants on your wish list, then you have found a good one.
Getting to know how quickly plants reach maturity will help you to plan your expenditure. If you wish to purchase Eucalyptus, for example, these trees are so fast growing that to spend money on a large plant is pointless. On the other hand, palms are painfully slow growing and to buy a very small plant, hoping to have it as the centrepiece of your garden one day is a really long term project that could be frustrating until it happens. Generally, if a plant is reasonably quoted, that tends to mean it is fairly fast growing, and plants that seem expensive are usually slow growing.
Something else to consider before working out a budget is the size of a plant in relation to its hardiness. For example, desert plants like agaves can cope with the colder period of the year only if they are at least 60cm in height. As a general rule, it may be worthwhile purchasing a mature plant if a younger plant is less likely to withstand the conditions in your garden.
What to avoid
Plants with any of the following traits should be left on the shelf:
Small, undersized foliage.
Leaves that are brown, yellow, or shriveled.
Masses of roots hanging out of the bottom of the pot.
No sign of any roots at all.
Considerable leaf-drop.
A pot so weed-ridden that the plant can only just be detected.
Pots containing completely dry potting soil that clearly hasn't been watered for a very long time.
The perfect specimen should be weed-free, have been recently watered and most importantly, should have leaves that are bursting with health, with few roots showing at the bottom.