Lancaster Farm Markets
Most of the places I would recommend are closed on Sunday. You can find a decent variety of local smoked meats at one of the local supermarkets - Stauffers or Oregon Dairy. Dietrich Meats in Krumsville is open on Sunday. It's not in Lancaster, but a great place for smoked meats and cheeses if it works into your travels.
I agree with Ahab. No farmers markets will be open.
Local Supermarkets are your next best bet. Besides Oregon Dairy and Stauffers (now officially known as SKH), other local supermarkets would be Darrenkamps, on Route 272 South of Lancaster, and Yoders on Route 23 just east of New Holland.
Dietrich's would be an excellant choice if you are travelling I-78.