Rather than paying exhorbinant shipping costs or running all over town, you might consider making the sausage yourself. As has been pointed out, Lincolnshire sausage is just a sage sausage, not too disimiliar from American breakfast sausage. The original recipe calls for pork, pork fat, stale bread, water, salt, pepper and sage. Compared to sausage in this country, many English sausages are much higher in fat and filler. In other words, you can make a superior product at home, assuming you have either a meat grinder or food processor.
I would suggest the following proportions:
1 lb pork shoulder
a panade made by mixing 3-4oz of stale bread with water and squeezing out the excess water
1 T sage
1 1/2 t salt
1 t freshly ground pepper
sausage casings
Cut the pork shoulder into 3/4in cubes, and place in the freezer for 20 minutes. Either grind the meat or use a food processor fitted with the metal blade to pulse the meat until finely ground. Stir in panade, and seasonings. Use a sausage stuffer, or pastry bag to fill the casings.