My mom had a big problem when I decided to become a vegetarian. I guess I got to her because she's an animal lover. I just explained why I felt like I had to do this (Vegetarianism) and that I hoped she would respect my choice.
I used to offer to cook dinner every night, so my mom would take me shopping, and then I would just grab everything I needed to cook the meals I'd planned. My mom was pretty amazed at the meals, and glad she didn't have to cook every night.
If their problem is that they can't be bothered getting you vege sausages etc, maybe just go shopping with her and grab them yourself. If she really has a problem buying them, maybe pay for it out of your own pocket while she's there. It might make her warm to the idea of you deciding to become a vegetarian, it also might make her think that if you're willing to fork out for it yourself, you truely believe it's the right way to live/right thing to do.
Alot of parents don't understand because they were brought up eating meat.
If it's because they're worried about your health, get them to accompany you to the doctor, so they can learn about your dietary needs, and that it's a healthier lifestyle choice.
Good luck, and stand your ground