Do you want PS2 models specifically, or just higher quality models for the grunts? The grunts in the HD pack for Half Life on PC are actually already higher quality in terms of texture sizes and poly count, though missing fingers for some reason...
Alternatively, you can use Ambient.Impact's models here: http://old.ambientimpact.com/downloads_npc.htm
Or for even higher quality there's Romka's models here: http://old.ambientimpact.com/downloads_Romka.htm
Otherwise, I'm not sure if the PS2 models specifically have been uploaded anywhere.