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