One way to get art for your project is to do it yourself. This however is not recommended if you don’t have any previous knowledge in this field and / or don’t know how to use a graphics design program such as Photoshop (Or free graphic design software such as GimPhoto).
If you do however, this is the cheapest way to go about it and potentially the best, as you will have full creative control. You can try out as many ideas as you want and tweak it till you get it just right.
Using free graphic design software can well will of course help keep down costs, as programs such as GimPhoto and Paint.net can be downloaded and used for free. While Adobe Photoshop it more expensive, it’s currently seen as the best graphic design software around.
Having said that, if you don’t know how to use graphic design software and aren’t willing to put the time in to learn, there’s not much point even if you can get free graphic design software. In that case it’d be better to use a graphic designer.