If you can't find the perfect rocks, you still have options.
Find flat but pourous rocks, and use spackle to make them smooth.
Visit a quarry or landscaping store. You can get a lot of the right type of rocks for pretty cheap... you pay by the pound!
Check the garden section of a home improvement store such as Home Depot or Lowes. You can buy a large bag of river rocks for about $10. Not all of them will be flat, but you can find other ways to use the misshapen ones.
Buy river rocks from Amazon. This is a more expensive way to get rocks, but the rocks you get will be perfectly flat and smooth. This is especially a good idea if you need rocks for a special project, like a rock painting party or making holiday gifts.