I am quite lucky in that I always pick our plants from the nurseries. We have left it to the growers to deliver unseen, but we were never satified with the quality. The quality of the plants we have to use for garden shows have to be gold medal standard or the potential for us to nurture up to RHS acceptability. Even with the gardens we do for clients, if I cannot grow them myself, I go to the nurseries and pick them out. It can sometimes mean driving hundreds of miles just for a few plants. But our clients are prepared to pay for the best so it the best they get.
Search your area, it is easy now with the internet, you will be suprised at the ammount of good little nurseries in your area. (Wigan might be an exception) ;D don't be put off by being (trade only) an allotment society should be able to open an account with the wholesale nurseries and get the plants you want at the price you want to pay.
Just for example a tray of 120 herb plugs at between £10 and £12, shrubs in 2 litre pots £3 to £4 the type that you would see in a garden centre at £15 and don't forget you see the plant and you do not pay carriage. I got some espalier trained fruit trees yesterday at £8 each 3 years old well looked after. £35 each if I had to pay retail prices. Just as you do when you get your groceries. Shop local, these guys need your business.