I have ave done much successful plant shopping on line, especially if you want something special then often a specialist nursery that deals mostly in that plant is the better option, many of those deal on line and I have found them very good. The more general catalogues are very variable, some of the most commonly found in magazines are often very dodgy. If you can find a good nursery where planting, cuttings and sowing gets done on site, this is the very best bet. For ordinary bedding, especially summer bedding, decent garden centres and larger supermarkets can be a good bargain. Outdoor markets can be good, but often the plants have been kept indoors and get very shocked when they suddenly find themselves outdoors, especially in colder or windy weather.