Tom- find a NAPA in your area that has a truck parts section (usually they have a separate counter for the truck section, w/air brake parts, truck tire changing gear...). They will have the stuff. Also a local Freightliner service center will, and maybe at pretty good prices as truckers tend to be a frugal and well connected lot so to sell much Freightliner shops have to be competitive.
If you start at the Alpine Owner's Forum page and do a "FIND" on "SCA" you will find all the info you need,
and more, for changing ISC engine coolant, including capacity, advice on filters, alternative (OAT) coolants, Fumitomo valve replacement for your drain plug, containers & process for oil & filter swaps, blah, blah,...
If you do a find on "coolant" you can probably read posts for longer than it takes to change the coolant