I didn't think that Tikka Masala contained coconut but according to Wikepedia, sometimes it does.
I was going to suggest that you make your own sauce but I have looked at dozens of recipes and none contain coconut.
I take that back, I found one:
For the Masala Sauce
7 oz (200 g) of tinned tomatoes
4 fl oz (115 ml) of natural yoghurt
4 fl oz (115 ml) of coconut milk
2 chopped green chillies
1 sliced onion
3 crushed cloves of garlic
2 tbsp of ghee
2 tbsp of lemon or lime juice
1 tbsp of black onion seeds
1 tbsp of freshly chopped coriander
1 tbsp of freshly chopped mint
� (half) tsp of garam masala
To make the masala sauce, heat the ghee on a moderate heat in a medium - large sized pan. Add the onion slices and cook gently for about 5 minutes.
Add the black onion seeds, chopped chillies and the crushed garlic cloves and stir the ingredients together with the onions. Cook for 2 minutes.
Then add the tomatoes, natural yoghurt and coconut milk and bring to the boil.
Reduce the heat to moderate and simmer gently for 20 minutes stirring occasionally.