Crate & Barrell, Pottery Barn, Williams Sonoma, and maybe even Starbucks. Crate & Barrell probably has the cheapest set, but I'm not sure they have porcelain. Williams Sonoma has nice porcelain espresso cups. I know that for sure.
If this person is really into espresso, they might want some Illy cups. They're collectors items for a lot of people, but they're somewhat the quintessential espresso cup design as well. I'm not sure where you can find these in the store though.