I'm sure you already have your preferences when it comes to buying books, whether it's online or on the highstreet, but anyhoo, here is a list of a few bookstores that TF has affiliation with, that if you chose to buy your books through, you would be helping to support TF.
www.computerbooks.co.uk
www.worldbooks.co.uk
www.countrybookstore.co.uk
www.dorlingkindersley-uk.co.uk/
www.play.com
www.whsmith.co.uk
www.amazon.co.uk
www.abebooks.co.uk
www.bookcloseouts.com
www.ebooks.com
www.ebay.co.uk
www.questia.com
www.iseeme.com
www.listen2books.co.uk
If there any other online bookstores that you use, please PM me the details and I will make an effort to have them added to this list.
Obviously this list will be updated and more will be added, as I find them and as I make relationships with them.