With a lot of patience and picking out of the best stock. Try another LFS or ask yours if they can get any other colours in.
If you are really determined, pick out the fish which have the characteristics you want, then breed them. Separate out the fry as soon as it is possible to tell them apart (before they breed) and raise them in separate tanks, once they are mature and showing their full adult colours, pick out the fish with the characteristics you want and breed those. Repeat until you have the colour you want. This process takes years and usually requires at least 9 tanks. If you do not cross the strain out with unrelated fish, your strain may end up having health problems because of the inbreeding.