Heroics and Ros and Ground Zero Games probably have something that would work. However, navigating GZG’s website for 6mm Dirtside figures is terrible. At least you can look at some of the 15mm or 25mm figures and determine if the same 6mm figures would work.
I would recommend Dark Realm if they still exist. There infantry were excellent, I have a good size guard infantry regiment with the their figures. Hope they still are going, but the website has been in a build phase for years sadly