Try Amazon resellers. Sometimes you can pick up nearly new -1 editions for pennies (often the postage costs more than the actual book!).
Also bundle deal s on Ebay where you pick up several books rather than just singles.
I would say that about 50% of my library was acquired in this way and you will save yourself a fortune.
Unfortunately there is no cheap way to acquire the tax books which always need to be the latest version.
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Shaun
Responses are not meant as a substitute for professional advice. Answers are intended as outline only the advice of a qualified professional with access to all relevant information should be sought before acting on any response given.