where can i buy all sorts of shiny and coloured pokemon?

1.) Promos
2.) Auction House
3.) Post in forums that you'll buy a colored Pokemon or maybe search in forums on people that are selling their colored Pokemon.
4.) For Shiny Pokemon. You can get it in random swap or a chance in lucky dip. Also right now in advent calendar.

-I hope it helps. and also, you need coins for buying colored Pokemon.

And brys,you missed out one,
Finding them on maps.
You can find them but you have 1/300 chance of encountering one in the maps.
I've found two on the maps already but you need patience to find one