Where to find ads

I have a forum website and placed google ads but google rules requires the ads to be placed on the places where there is "live text" so I think that I cannot place ads on the right side bar. Could you please suggest me companies that accept the ads to be added on the side bars? Likewise, since my forum is a mechanical engineering forum, could you please let me know how to choose ads to be only with relevant issues?
Thanks

See original post by klwm