The things I have noticed with grey colored ball pythons is either they are really expensive, many morphs in one or you are looking at recessive genes like the lightning pied... which is gorgeous by the way... but you are looking to spend quite a bit of money on one of those. Really, Axanthic is your best bet for grey BPs, and honestly, the breeders at repticons or reptile expos are very well known and wont lie to you, they really have no reason to. I am not sure what type of breeders will be there as I am not in the US but I am sure people in your area will know who is who.
But I know of a couple greys that may be a little cheaper and no axanthic like the pewter or pewter bee. Although they do have some creams an browns. Most wont stay super grey but I have seen some pretty good quality ones that do.
Black Pewter as well is a really nice choice. Again they can brown out.
They is also the grey matter which looks pied but is actually a super cinny champagne.
That is all I can think of for now, but I really think if you want grey, go for axanthic.