Do them yourself! If you plan on making a serious game, you're not gonna get anywhere by just depending on others. That's one of the reasons game developers rarely team up. You see, things like this is hard. Way harder than what it seems like at first.
That's my tip. It'll take you a year to become a spriting pro, but then you'll be in a way higher position than where you would have been if you had relied on others to do the hard work! You're already a skilled mapper, why don't you learn to be skilled in spriting?