Amanda Ackerman is quite correct. All lactose, from whatever 
source, is identical. However, according to my records, the 
lactose content in cow's milk varies considerably, from 3.7-5.1%, 
thus overlapping the lactose content of goat's milk,
which is from 4.1-4.7%. 

There is no easily attainable milk that is low enough in lactose
 to make a difference, and the really low lactose milks 
(sea lion, eg) have so high fat as to be unpalatable.

For a long list of the lactose percentages in the milks of various 
animals, see the Lactose Zoo page in my web site, Steve Carper's 
Lactose Intolerance Clearinghouse.

Steve Carper
http://ourworld.compuserve.com/homepages/stevecarper/zoo.htm