Believe it or not, Lopez Bakery on 5th Ave. between 18 and 19th st.
While you're there for his challah, pick up a babka muffin and anything else that looks tempting.
Lopez Bakery is the unsung jewel in Park Slope's crown. He's been quietly preparing fine baked goods for years.
I was desperately looking for challah to make French Toast with once and found one at the French bakery on the north side of 9th street between 5th and 6th (can't remember the name.)
Les Delices de Paris. Their challah is a cross between challah and a croissant. (I'm a huge proponent of this place in most all other areas, though....great cakes, pastries, crazy-good almond croissants, lattes, etc.)
I was just thinking today how incredibly shocking it is that there aren't more places to buy challah in the neighborhood. They used to sell a good one at the old Uprising.
Eagle Provisions on 5th Ave. @ 18th St. has some nice challah, too. Not baked in-house or anything, though, but good.