This is a frequent problem that game developers encounter that's hard to surmount. Unless you can get someone on-board full-time for free (like a partner on your development team), I would really recommend learning how to create basic artwork yourself.
With tutorials and basic stuff available on the internet (sprites, textures, icons, etc.) you can quickly learn to create polished technical art -- not the most beautiful, but good enough to launch.
I also personally find that Flash is a great tool for creating (vector) art assets. It has a simple but sufficiently diverse toolset to allow you to create good technical art. (Technical means not necessarily creative or polished or stylistic; but technically clean.)