A Google search shows that ToonHeads is on TV.com, a CBS-owned legal streaming video site similar to Hulu. Unfortunately, the site is partially down for maintenance at the moment, but check back later and you can probably watch at least some of it there.
I assume you're asking about LEGAL ways to view the show online, because linking to, or even asking about ways to obtain illegal copies of copyrighted material is strictly verboten on these forums, so if you were, I'd have to give you a warning. I don't like giving out warnings, so I'm going to give you the benefit of the doubt and assume you're staying legit.
Also of interest to you may be an episode guide I found listing the shorts from each episode. You can probably find many of these shorts on DVD: most of the Looney Tunes ones, for example, are on one volume or another of the Looney Tunes Golden Collection.
