Wow, I've never had Weißwürst in the traditional Bayern manner! I think you can buy them all over the place, but they don't tend to be very good. The European Deli (vendor 1100) and Swiss Pastries (vendor 1299) have them for sure. Have you tried theirs? I'm reluctant to link this food to a vendor until we know where the good ones are. I don't see them listed on the product page of Bank Street Sausage shop (vendor 1188), but there are certainly white sausages in the photos on their homepage: banksausagedeli.com
We bought these sausages at the Lansdowne Farmers Market last weekend. Put them on the BBQ, didn't take long, with some BBQ sauce and WOW, these were really really good sausages! We are very picky about the meat we eat, since it is so rare, so we choose pretty wisely and don't mind spending a little more for quality since it's not that often. These were mildly spiced and very lean. And surprisingly for a sausage, quite light! Didn't get that heavy feeling in your stomach afterwards...and I ate 2!