I started making my own Coconut Milk in the past month. All the store bought canned coconut milks have guar gum or some other additives to keep it emulsified. I wanted a pure coconut milk. This is super easy and cost way less than a can of coconut milk to make. I use this same coconut milk to make Coconut Milk Kefir.
Homemade Coconut Milk
1 cup of Shredded Unsweetened Coconut (preferably organic)
1 cup of boiling water (should be filtered, or distilled)
Pour boiling water over shredded coconut and let sit until cooled. Pour mixture into a food processor or blender. Blend until very smooth. Pour blended mixture through a cheese cloth (I just use a muslin cloth). Squeeze the cloth to extract the liquid. Keep squeezing until no more liquid comes out. Discard the pulp. Store your coconut milk in the fridge. Use within 2-3 days. If you let it sit beyond about 4 days it will go sour.
I generally try to only make what I can use within a couple of days. It also freezes nicely but separates a little when defrosted. You can just blend it again to bring it back to a smooth consistency.
I buy my shredded coconut in 5 lb bags for about $16 and use it constantly.