Tushantin was the animator. I have no idea if he does commissions, though. It's always worth a shot!
Also -- your wife has great tastes! Howl's Moving Castle is my favorite Ghibli movie (tied with Spirited Away), although I do like the book a bit better than the movie (they just diverge tonally so much in the second half, and the book is so much lighter and more fun - plus it all ties together better with the revelation to the Witch's schemes).
As for Castle in the Sky (aka Laputa) by Studio Ghibli, it's actually entirely separate and not related to Castle in the Air (the book) at all. Similar names, but no relation. The book is more of a semi-sequel in that it takes places in a new, Arabian country (not Ingary, the location in Howl's Moving Castle) and follows entirely new characters, and towards the end do find out that two of the characters were Sophie and Howell transformed.