I'm not sure what you mean by 'I want to buy a family tree'. You don't just 'buy' a family tree.
Do you want to do the research yourself and just buy a chart to fill in or do you want a ready done tree? If the second option, you will need to consult a professional researcher which can cost anything upto �500 for a tree, depending how many surname s you want researching and how far back.
You say that you want an easy to use one which suggests that you want to research yourself - can you clarify please?