Is there a replacement for meringue powder in royal icing?

Pasteurized egg whites are a practically perfect substitution. They're available in the refrigerated section of supermarkets. If you make a lot of royal icing, the other practically perfect substitution is egg white powder, which is just dehydrated albumen. Deb-el's "Just Whites" are wonderful and their website has a very helpful quantity reconstitution chart. http://www.debelfoods.com/recipes_justWhites.htm

fresh egg white is probably the easiest substitution, but if powdered egg whites (more likely found in a grocery or health food store than meringue powder) should also work.

You can make royal icing with fresh egg whites, instead of meringue powder. Here's a link with both recipes:
http://www.joyofbaking.com/RoyalIcing.html