If a plant is doing well in a neighbouring garden there is a fair chance that it'll do well in yours. As noted elsewhere, an excellent way to see local gardens is through the National Gardens Scheme. Under the scheme hundreds of private gardens are opened to the public for a few days each year.
Visit garden centres. You will find that plants are labelled with information on the type of conditions they prefer. There should also be information on the eventual height and spread of the plant. Another useful thing about reputable garden centres is that they usually have someone on hand to advise you and will normally give a guarantee on most shrubs and trees.
Recently there has been a resurgence of municipal planting schemes. Not only town parks but roundabouts and verges are being transformed into a riot of seasonal colour. I know of one local council which sells surplus bedding plants which gives you access to local plantsmen.
Buying plants has never been easier. Nurseries will send you glossy colour catalogues. The Internet yields a plethora of businesses offering plants by the score. Supermarket and DIY chains offer a wide variety of plants all year round. For we consumers this means competition and lower prices but there are drawbacks. Look for sustainability. Have the plants been sourced from commercial growers or have they been harvested from the wild? Are you just buying an exotic plant or are you contributing to the unwitting import alien plant diseases and garden pests.
At the risk of upsetting people I always advise customers to avoid car boot sales unless they are sure of what they are buying. What seems a dainty little plant when you buy it could be a monster in a couple of years and although digging it out is always an option it is a waste of energy and money.