I am still fairly new at this. But I like Baker's Creek Heirloom Seeds, Pinetree Garden Seeds, & Territorial Seed Company. Territorial has a special section on their website that has winter veggies. Sorry I don't know how to provide links. Maybe someone else can.
Kendra

Seeds of Change has a number of lettuce selections that are supposed to survive well in freezing temperatures such as a hoop house or cold frame. I have also bought from Shepherd's Seeds but just about any seedsman in the northern areas will have things that will do well in a winter greenhouse.

MOgardener--not to be confused with MOgardener1

I too used Seeds of Change and The Cook's Garden. Both have seeds that are heat and cold tolerant and worked well in my cold frames inside of greenhouse this past winter.

I really like Territorial for their fall catalog. It is exclusively for winter gardening and describes items with do best when planted in the Fall. They offer excellent varieties and provide solid growing advice.