A great site for high quality yet affordable wildlife photos is www.wildlifereferencephotos.com
. I'm co-founder of this site and we get around 1000 new photos added every month. We have many talented photographers from all over the world contributing photos, and all of the photos submitted are checked to ensure that only the best make it onto the site.
Also, when using any of the photos as reference for your artwork, you have permission to sell that artwork and also any prints/reproductions produced from it. If you wish, you may also enter your artwork into competitions, display it in galleries or exhibitions, and have it licensed. Basically you have the freedom to use your artwork in whatever way you would like with these photos.