What you can do is have a look on the internet to see what colours are soild and not a mix, then you can have a look on online shops, or find local breeders in your area who will be able to give you healthy guppies for breeding.
Note: when you do breed them make sure they have homes to go to, or your LFS takes them! I have about 30 in my community they are still small though.
Good luck!!!
Sean!!