http://librivox.org/ offers free audiobooks read by volunteers. The books are all in the public domain. Also, try http://etc.usf.edu/lit2go/ – it’s a collection of free audiobooks provided by the University of South Florida. I got some of my favourite ones from there.

Cheers
S
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or amazon can be quite cheap:
http://www.amazon.co.uk/s/ref=nb_sb_ss_i_0_5?url=search-alias%3Daps&field-keywords=audio+books&sprefix=audio
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