third-party wide angle lens for 710IS

[dstudio2007]

Hi, I would like to get wide-angle third party lens for my 710IS. Any recommendations?
I believe that wide-angle lens will have distortion at the sides. I tried a 18mm one and the distortion was very bad. How many mm would it be ok?
Is there a way from photoshop elements that I can solve the distortion problem?

Thanks