The smallish black pebbles/cobbles are usually called "Mexican Beach Pebbles." Is that what you're looking for? They used to be available at many places outside the DC area, but I haven't seen them for sale for years now. I'd start by trying Luck Stone.

Yes, they are from Mexico. I believe I saw them either at Betty's Azalea Ranch or Merrifield. I bought my from a vendor outside of Atlanta who sells granite Japanese lanterns.

Did you ever find the source you were looking for?

Hi, there! Yes, actually, I did -- I found them at Irwin Stone in Rockville, and I bought them there a few weeks ago. (Actually, we need to go back and get a few more bags.) They call them Mexican Beach Pebbles. Here's their web site, they also have a branch in Frederick:

http://www.irwinstone.com/

When I was there, they only had the plain version, not the resin coated. But they are very nice, and come in several different sizes. (Don't buy them all up, I still need some! ;) )

Jenny