hmm, there is a d20 mecha book out there, dont recall who prints them at the moment tho. allso, the biggest thing about anime is the drawing style, other then that its often quite thesame consepts that you find allready, fantasy, sci-fi. cyberpunk, kids with insane martial art skills and powers (alltho i dont know of a d20 book that covers that). so unless your looking for books tied specificly to a anime/manga series then you most likely can pick up any set of d20 books and replace the artwork to get a anime feel...
edit: go fig, the people beind d20 mecha re allso the ones behind besm (and its d20 offshot)...