Why would you want that?
Manga written in only hiragana are pretty rare. There's a reason why the language uses kanji, sentences with only hiragana makes it harder to see instantly what word it is, what verb is really being used and so on.
You can read lots of manga anyway if you only know hiragana, because there's hiragana besides the kanji (called furigana) which are there to show the reading.
Especially shounen manga has lots of furigana, I'm reading One Piece and it has furigana for almost everything.
Anyway, it depends on what you want to do, if you read hiragana manga you'll get really good at reading hiragana, but "real" Japanese isn't only hiragana. If you read normal manga with furigana you'll still be able to read it, but you'll also be able to see the kanji, maybe you'll be able to read lots of kanji just by having looked at furigana a few times. Also, normal Japanese uses kanji a lot, so getting used to it early would probably be good.
Sorry that I didn't really answer the question, but there should be plenty of websites selling manga with furigana.
http://www.google.com/search?hl=sv&q=manga+with+furigana&btnG=S%C3%B6k&meta=