MW 4 Mercs game can be purchased from a variety of places
and is Mandintory for the combat engine of ISW at this time.

The compilation pack has all 3 games Vengence, Blacknight, & Mercs

Mercs is all that you will need for ISW, but you can have all 3 installed
Infact you can have multiple Merc installs for those that play in leagues, if you do
and are unsure how to have more that one and get them all to work
talk to ColoKid or Aylward one of them can help you.

check at your local gaming software store or

Ebay has some MW4 Mercs listed

I am sure there are other places online as well