http://www.j-bonsai.com/ A famous online website from Yoshihiro Nakamizu, he is been in the business for decades and beside teaching and educating students, he does offer Bonsai artworks online.
http://www.bonsaiboy.com/ Online store from New York, they grow a huge assortment of Bonsai trees. Although I have never purchased directly from them, it's worth to check. Please note that they do not ship internationally, US only.
http://www.kaizenbonsai.com Another fantastic online shop, Graham is very popular on YouTube. The way he wires and shapes Bonsai in his workshop is simply remarkable. Another good point is that they ship worldwide.
http://www.bonsaioutlet.com/ The most popular online store website in the US. They sell all kinds of Bonsai sizes and accessories. They ship to US only and not internationally. Very competitive pricing and fast delivery.
And if you still haven't found your Bonsai, then try on YouTube, artists are often very helpful to introduce you with the right counterpart. Most of them do it out of passion and not necessarily to make a living. Good luck!