Here are providers of Slovak and Eastern European food in the local Washington, DC area as well as few online sources.

Markets

Polish Market
431 Maple Avenue West
Vienna, VA 22180
(703) 261-6385
https://www.polishmarketviennavirginia.com
Products: Kiełbasa, pierogi, Gołąbki (stuffed cabbage), Pickles and Sauerkraut, Kosciusko Mustard, Farmers Cheese, Pączki (Polish doughnuts), Makowiec (poppy seed cake), Żywiec (Polish beer).

Troika
(Russian Market and Restaurant)
169 Hillwood Avenue
Falls Church, VA
(703) 241-3777
https://troikaonline.net
Products: Farmers Cheese.
Menu: Stuffed cabbage and vareniki (pierogies).

Kielbasa Factory
(Polish & European Deli and Grocery)
1073 Rockville Pike (Talbott Center)
Rockville, MD 20852
(240) 453-9090
http://kielbasafactory.com
Products: Kielbasa, pierogi and other Polish and European specialty goods.

Polka Deli
(Polish Food and Market)
13438 New Hampshire Avenue
Silver Spring, MD 20904
(301) 384-DELI (3354)
https://www.polkadelimd.com
Products: Pierogi, sandwiches (e.g., breaded pork chop), groceries.

European Delight
1488-I Rockville Pike
Rockville, MD 20852
(301) 230-9371
Products: Russian, European and International food, Cookery, Delicatessen.

Restaurants

Bistro Bohem
600 Florida Avenue, NW
Washington, DC 20001
(202) 735-5895
https://www.bistrobohem.com
Menu: Halusky, pierogi, schnitzel, smažáčky, Czech, Slovak, Polish beer and liquor.

Online

Andy’s Deli
5442 N Milwaukee Avenue
Chicago, IL 60630
https://www.andysdeli.com/online-store-catalog/
https://www.andysdeli.com/product/polish-sheep-cheese-bryndza/
Products: Bryndza, Polish rye bread, poppy seed cake, pierogi, etc.

S&D Polish Deli
2204 Penn Avenue
Pittsburgh, PA 15222
http://www.polishfoodandgifts.com/bryndza-sheep-milk-cheese/
Products: Bryndza, pierogi, etc.

Fabko Online European Store
210 Trumbull Street
Elizabeth, NJ 07206
(908) 576-6265
https://fabko.com/278-czech-slovak-food-deli
Products: Czech and Slovak (and other Central and Eastern European) food.

Butter Maid Bakery
425 Boardman Canfield Road
Boardman, Ohio 44512
1-844-NUT-ROLL (688-7655)
https://www.buttermaidbakery.com
Products: Kolachi rolls and cookies, and lady locks.