I'll third on Blambot. Great selection, and the guy who runs it just oozes professionalism to boot.
I use his Digital Strips font for my lettering, and I kind of wish I didn't, because I get the impression that everyone and their seven-faced hyperdonkey uses it. I've been using it for the majority of my current project's life, though, so, for consistency's sake, I'm kind of stuck with it. No great, horrible deal, but it's one thing that I wouldn't mind changing. There are plenty of other fonts out there, after all.
For onomatopoeia, I just do it by hand with the tablet. On a light background, I'll use black with a thin, white outline. On darker backgrounds, white with a black outline. I also keep sound effects on their own layers and set them to 75% transparency, which helps to subdue the color a bit by forcing it to conform to the general color scheme of the frame. If you really want to use a font, your best bet is to just play it by ear. Again, Blambot gots the goods for you.
