The best source for a Ball python would be a reputable breeder or good reptile show. If you're in SoCal, there's a whopper of a show coming up in SanDiego you could check out.
As for the mentioned enclosure, if you got one that 'large' and ended up with a baby snake you would have to add lots of hides and fake plants or crumpled newspaper to reduce the open space and make it feel more secure. These snakes don't like wide open spaces and can get stressed out if in a really big open cage.
Every snake handles things differently, but I think if you made sure to add lots of (safe) decor to fill the space and make sure you can get the proper temperature gradients, then the display cage should be alright.