Simply finding an artist to work on the cheap, or with the promise of future royalties can be a monumental hurdle in itself, let alone an artist that you can direct as you wish.

BlackWhite has the right idea. If you're just starting out (as I assume you are, and I apologize if that's not the case) and you're on a tight budget then be prepared to make concessions. Perhaps even a lot of concessions. You're likely not going to find a professional-grade artist who is well versed in multiple styles unless you're prepared to offer a professional grade of pay.

You also might not be able to find exactly what you need as far as style or level of detail goes... but the most important aspect to a game's visuals isn't necessarily a high degree of technical aptitude, but consistency. If you can find an artist who isn't perhaps the best or most fitting, but is able to produce consistent work across the board, then that in itself will go a long way to making your game look good. I've played quite a few low-fi indie games with chunky, childish art made in MSPaint that looked quite charming and stylish, simply because the art was consistent throughout.

Also, you may have a vision of how the game should look floating around in your head, but you might consider loosening the hold you have on that vision. If you can find an artist who can make that vision a reality then more power to you, but letting a [s]more amateur[/s] [s]cheaper[/s] [s]inexperienced[/s] budget friendly artist express themselves in a style that they're more comfortable with will produce better results. After all the goal is to get the game done, yes? If "just the right art" is holding that back, then as harsh as it seems it may be time to reorganize your priorities.