You can get tiny small potatoes at Trader Joe's (called fingerling potatoes). I just coat mine with olive oil, sprinkle salt on them and some rosemary, and cook them for about 25 minutes on a roasting pan at 400 degrees. Delicious.

The small round ones OP is talking about aren't fingerlings, they're round--about half the size of golf balls. I call them marble potatoes. (I think that's what they're called on the menu at Jaleo.)

But the above method is exactly how I cook them. Or sometimes I fry them with a little oil in a skillet. You have to either fry at a lower temperature for a longer time than you otherwise might, to cook them all the way through, or you can parboil them for a couple of minutes first. I can never decide which is less bother--roasting or frying.

They have them at Whole Foods and sometimes at farmer's markets.

Whole Foods has those, I believe. They are delish!