I use a combination of Matthew's Google search method AND a few LinkedIn groups I belong to that focus around either my geographic location (a big part of my business exists off-line) and/or my key focus areas.

Also, the easiest thing you can do to get your comments approved (IMO) is include the blogger's name and 1 key point from the post itself.

Addressing someone by name and actually making a valid point? Priceless!

PS I've also struggled with the "poof, my brilliant (read: lengthy) comment just disappeared!" thing - thanks to the wonderful Denise O'Berry... I now copy EVERYTHING before I hit 'submit' - that way I can try one more time. If it happens again, I figure the issue is with their spam blocker software - and I move along...