I believe that the challenger series are FIRESTONE kegs, technically, not Cornelious kegs. This distiction is really only important when buying new posts or poppets (and occasionally blowoff valves, depending on the lid style). This is mildly confusing because the soda kegs we use for beer are generically referred to as 'Cornelious Kegs' when, in fact, that is only one of the brand names that makes items for this style of kegging system.
I've never seen cornelious posts for firestone kegs, the threading is different, IIRC. Your 2 best options are a) keep an assortment of corney and firestone parts on hand or b) find a brew buddy who has Cornelious and Firestone kegs and swap kegs till one of you has all the Cornelious and the other has all the Firestone.
I just got one of these blue top kegs also. It has a plastic gas side dip tube in it. Does anyone know if a regular SS dip tube will work in it? Or should I just use the plastic one. It seems a little nasty.