I don't find Amazon too bad, but I guess it depends what exactly you're after. As well as Book Depository, Wordery and Abebooks both have a pretty good selection and are usually a quid or so cheaper than Amazon and BD (so far, not always, deff worth checking all four) on in print decks. For out of print, Abebooks seems a little more reliable in terms of actually having stuff, but if Wordery or Book Depository have it they may be a touch cheaper. Afaik Abebooks and BD are both owned by Amazon now, but for whatever reason BD often has a different price even for things which appear to be the identical item, available from them but through Amazon marketplace. Abebooks is pretty much made up of lots of sellers of different sizes, I think, and not all of them list stuff on Amazon marketplace, so it's usually worth checking both, plus Abebooks sellers can offer free/specific priced postage whereas marketplace has a flat £2.80 shipping fee.
If a deck is published in the UK, it may also be worth checking the publisher's website, e.g. I bought my Wildwood direct from... Either the publisher or the author, can't remember, but anyway, very nice people, little cheaper than Amazon I think it was. Watkins publish a few decks, though I think more Oracle than Tarot, but they're worth checking out (they also have a shop in London - I think it's the same people, anyway.)