This article is from the Care of Orphaned Kittens FAQ, posted to rec.pets.cats newsgroup. Maintained by Sharon Talbert with numerous contributions by others.
There are several good milk replacers on the market, available in liquid or powder form (my personal favorite is called Just Born). The ready-mix liquid is more convenient. Be sure the product is engineered for kittens and that it is fresh (some have a short shelf-life). Milk replacers can be found in any pet supplies store, most veterinary clinics, and even in some variety stores. In an emergency or for the short-term, you can make up your own formula from tinned or powdered goat's milk (see below). If the kitten seems weak or ill and you cannot get to a veterinarian right away, you should administer slightly warmed Pedialyte before offering the milk replacer.
Mona's Homemade Goats Milk Formula
Mona Myers, a certified bird rehabilitator in Seattle who has in the past rescued orphan kittens, swears by this formula and prefers it to the ready-made products. You might try her recipe if the kitten is not responding well to the commercial product.
Use tinned or powdered goats milk. (Either should be kept in the fridge when opened>) For a newborn or a kitten suffering from exposure, substitute Pedialyte for water to reconstitute the powdered goats milk. (Stick with the Pedialyte formula for the first week or so with a weak newborn, then switch to boiled water as the base.) Warm a measured amount of the liquid slightly and pour into a bowl. Using a flour sifter, sift the goats milk powder into the liquid, blending with a wire whisk. To every 8 oz of goats milk, whether tinned or reconstituted, add 1/3 dropper Avitron and 1/3 dropper Avimin (available in pet supply stores). Finally, add 1/4 tsp acidophilus culture and 1/4 tablet (crushed) papaya enzyme (these last ingredients are found in health food stores; acidophilus culture must be refrigerated).
This formula is best after being refrigerated for at least an hour, but it can be warmed (in hot water or microwaved a few seconds in a dish, not in the nurser) and served immediately.