I've just been faced with the same question myself, and after much research have found that either raw goats milk, made in a home-made formula below, or Nanny Care goats milk formula (http://www.vitacare.co.uk/) to be the best option. Goats milk is the closest thing to human breastmilk in its nutritional make-up, but you can also use raw cows milk to create the home-made formula.
Homemade Milk Based
Makes approx 500ml
2 cups whole, raw milk
1 cup filtered water or coconut water
1/4 cup liquid whey
1/4 tsp BifidoBacT probiotic (http://naturaldispensary.co.uk/products/Bio_BifidoBacT_75g-3695-181.html)
1/4 tsp2 caps Saccharomyces Bouladii probiotic (http://naturaldispensary.co.uk/products/Saccharomyces_Boulardii_50s-4295-200.html)
2 TBL good quality cream, raw if possible
1/2 tsp unflavored, fermented cod liver oil
1 tsp organic sunflower oil or hemp
1 tsp organic, extra virgin olive oil
2 tsp virgin coconut oil
1/4 tsp high vitamin butter oil
2 tsp gelatin
2 tsp Kefir
1/4 tsp vitamin C powder
Pour about half the water into a pan and put on low heat. Add the gelatin if used and let dissolve, stirring occasionally. When gelatin dissolved, remove pan from heat and add the rest of the water to cool. Stir in the coconut oil and butter oil until melted. Put remaining ingredients in a glass blender. Add the water mixture and blend for about 3 seconds. Place formula in glass bottles or a glass jar and refrigerate.
We have decided to go with the Nanny Care formula mixed with breastmilk.