Yeah, the best place is probably on Amazon, that's where I got mine. If you're willing to go used, you can get at least the first one pretty cheap. ($.01 plus paying for shipping you get it for about four dollars. Or at least, where I live shipping is $3.99, not sure what it would be to other countries.) I think the others are $12.99 new, plus shipping (not sure if you'd have to pay for each individually), though the shipping is free on orders over $35. (Only with Amazon-provided products though, so the TAI one wouldn't count, unfortunately, or any used items).
(Book 1: http://www.amazon.com/Artemis-Fowl-Graphic-Eoin-Colfer/dp/0786848820/ref=sr_1_1?ie=UTF8&qid=1442211184&sr=8-1&keywords=artemis+fowl+graphic+novel
Book 2: http://www.amazon.com/Artemis-Fowl-Arctic-Incident-Graphic/dp/B00CB22UR6/ref=sr_1_7?ie=UTF8&qid=1442211381&sr=8-7&keywords=artemis+fowl+the+arctic+incident
--Couldn't find a version of this one provided through Amazon, so maybe it's sold out or out of print? Looks like you'd have to get it through a second-party seller.
Book 3: http://www.amazon.com/Artemis-Fowl-Eternity-Graphic-Novel/dp/1423145771/ref=sr_1_3?ie=UTF8&qid=1442211242&sr=8-3&keywords=artemis+fowl+graphic+novel
Book 4: http://www.amazon.com/Artemis-Fowl-Deception-Graphic-Novels-ebook/dp/B00LIA181G/ref=sr_1_2?ie=UTF8&qid=1442211293&sr=8-2&keywords=artemis+fowl+graphic+novel
But your local library system is definitely a place to look, too. I know my library has copies of all of them (I got them from the library first before I decided to buy the collection for myself for posterity, lol, and to have on hand to use as potential art references), though I had to go online on our library's website and search for them, and put holds on them. (Because they end up at all different libraries within our branch.)
“After all, absolutely no one can help but suspect a criminal, liar, and manipulator of committing crimes, lying, and manipulating. And of course, no one is more aware of that simple fact than Artemis Fowl.”
Opal sets into motion her most diabolical scheme yet, to frame Artemis and turn his closest friends against him. Only this time she has a new calculating partner who knows Artemis better than he knows himself. [Post TAC] https://www.fanfiction.net/s/8336552/1/Noble-Heart
...Shameless self-advertising, guys! C;
(And if you're really bored: http://axxonu.deviantart.com/gallery/28912232/Artemis-Fowl
AF fanart. ;J)