So here's what I do.
search "Visitor Messages inurl:forum"
This usually brings up forums where they're discussing visitor messages so you know they must exist on the forum. Google just spat out who knows how many forum profiles where you can dump tons of links.
Register and view your public profile and you'll see the visitor message box and I'll dump about half, submit, dump the other half, submit. BOOM
These are mostly low PR but I do find a lot of high PR sites as well.
Sometimes you'll find forums where they've disabled them n such but that's to be expected.
You can get creative with this too if you know what you're looking for and can find a common keyword for forums that facilitate your specific need.