well you wont find them in an LFS and purple it depends how you classify it. there are lots of pink/purple boys in pet stores but when it comes to a solid lavender colour these are more rare
yellows are easy to obtain, but a true purple will reach a top price which will be in the 100s of £ or $ if genetic history is supplied that can be proven. But you never see them about as its a hard gene to maintain and does not spawn true.