Nutramigen is available in many places that sell regular milk and soy formulas.
Drug stores, supermarkets, etc.  You should not have a problem finding it here.

Sherbet is a milk/sorbet product and would not be safe.  Many sorbets sold here in
the US are made on the same production lines as ice cream products and therefore, may
be contaminated with milk residue.  I try to look for sorbets with a Kosher Pareve
marking on them.  There aren't many of them, though.

Soy is used very widely in products in the U.S. All I can suggest is that you read
labels carefully and ask detailed questions when you go to restaurants.  My son has
both milk and soy allergies so I know how limiting this can be when searching for
food.

Lynda