Grocery Stores in Central/South Park Slope
I used to live on that same corner. United Meat Market is great for steaks and the like. They're on PPW right next to Farrell's on 16th Street. Union Market is a rip off IMO. There are a few grocery stores on PPW between 16th and Prospect Ave, mom and pop shops. There's also a Key Food on Prospect Ave and 10th Ave. However, I've never been there so I have no idea if it's any good or not.
One of the best small mom and pop supermarkets in the city is Eagle Provisions on 18th St. and 5th Ave. Perhaps not quite walking distance, but well worth the trip. There is some great Polish, European and other international stuff as well as cheap meat, produce and bakery products, some very good, some ordinary.
C Town on 9th Street between 6th & 5th, the small supermarket on 7th Ave & 11th Street, Russo's on 7th Ave & 11th Street, Grab on 7th Ave between 14th & 15th, numerous corner stores & greenmarkets, Saturday farmer's market at Grand Army Plaza, Wednesday farmer's market at Bartel Pritchard Square, United Meat Market on PPW/9th Ave and 16th Street, Fresh Direct.
C-Town is sometimes too much of a schlepp for me. I'm lazy, it's often crowded when I have time to go because that's when normal people have time to go. That said, it's a pretty good neighborhood supermarket that is stocked in a way that seems to take into account the varying demographics that lay between 4th Ave. and the park.
With that in mind, I've become less of a stock the bomb shelter shopper and more of a shopper who cherrypicks from her favorite places as the mood strikes. There's a small market on 7th Ave. between 11th/12th (the one with all the flowers in front). Smaller than the market on the corner of 7th Ave./11th, but a nice little selection of various Asian sauces, frozen dumplings, and little snacky things like that. Small but well-maintained produce area.
Pumpkin's on 8th Ave./13th is an organic/health food store, but I never go there. If I want to spend that much, I go to Grab! Great cheese selection there, as well as what I would call a thoughtfully curated selection of gourmet foodstuffs. Russo's has a nice selection of pastas and sauces, fresh moz., and really nice sausage.
There are some interesting markets on 5th Ave. from 12th to about Prospect Ave. They carry a lot of Mexican/Central American produce and groceries.
OT: OMFG, when are they going to be done renovating the corner store on 8th Ave./12th??? They've been working on it since New Year's! I can't make an ice cream run to the sucky sister market at 8th Ave./11th and back during commercial breaks like I could 12th. This reno better be worth it!