I would contact a board certified veterinary nutritionist for recommendations. I use and would recommend Dr. Rebecca Remillard. She works with your veterinarian and the cost for her to review the records and formulate a diet is $250 currently. Her website is
PetDiets - VNC putting pets' health first!. You also can check out the
American College of Veterinary Nutrition website for other nutritionists. I'm not sure if there are any others in Canada.
LadyJane a member here has a dog with lymphangectasia and has a thread in the sick and injured section. You can search for it and read about her dog or just message her.