This is a Frugal Living contribution. I bought a Victoria plum very cheaply (£6 or 2 for £10) from Tesco this spring and had to take it back since, in order to fit it into the fairly small plastic bag required, they had cut off most of the main roots. But I learnt from the experience and deliberately chose a much smaller tree which had it main rootball intact and has established better than several apple trees I bought at the same time. Whereas the dry summer caused them to look decidedly unhappy, the plum has always looked in excellent health. The cheapest trees will always be supermarket trees but there are many specialist suppliers with reputations to preserve which will sell a much wider range of varieties at much greater prices and charge you commercial rates of postage on top. I am willing to take a chance on buying supermarket trees and over the years have done very well with them. If you are dissatisfied with the roots which you are unable to see when you buy the tree, you can easily return it at no cost. When you have bought a bare root tree, make sure you keep it for at least 24 hours in a bucket of water before planting and make sure if is kept well watered in the first summer and mulch it thereafter until well established and fruiting.