The housing terminates vertically directly below the flange the boot fits over, pointing straight up. Without that boot, there's basically nothing to stop water and grit from the rear tire from running down into the cable housing. Here's the view from underneath (brake cable on left, front derailleur cable in middle, rear derailleur cable on the right):


Derailleur cable housing
I'll check with an electrical supply house--that might be my best bet. GT's response was "It does not look like we have or stock that rubber boot separately from the bike. Our newer Grades do not come with them."