Blueberries are not the fastest growers in the world and if you succumb to buying a P9 size plant (usually £10 and under from most mail order companies), you'll be waiting 4 or 5 years for anything worth making into a cake!
Look for 2 litre pot size as a minimum. These are normally 3 years old already and will give you a small handful of fruit in their first season, if you can keep the birds off that is! Larger plants, of 3 litre and above, are more expensive but produce increased yields in a shorter period of time.
Looked after, in the right soil (remember, thats ericaceous.) your blueberry plants could be producing up to 3 kg of fruit when they are 10 years old.