[KeriVit]

Does anyone know where I can get that mottled/pebbled background pattern that is frequently used to black out parts of invoices and such?

Sometimes it's called Chinese- not sure why...

[eyevaan]

I don't know where to find the actual file but this is a pretty good workflow until then.
scan it
bring the scan into Flash
Trace image
Bring into illustrator -tadaah working illustrator file that you can duplicate as large as you want.

[Thorzdad]

Funny you should mention this. That is exactly what I was looking for the Pen and Ink filters for.
Use the Stipple filter and adjust the settings until you get the desired effect. I've used this many times to create just the sort of background you are describing.