Any home improvement store like Home Depot or a landscape supply / nursury sells them. They will come in a large bag, for use in planting beds - probably way more than you will ever need for a gaming table.
Another idea I learned from Pavlov Grenadier is to use a sea sponge. Home Depot, Target , wall mart etc. sell them. Simply rip the sponge into smaller pieces and spray it black or brown and drybrush it a lighter color.
They look great, and are a lot easier to haul around than Rocks! Easier to mount them on a base too if you want them floating above the table.