If a guppy sport is in a store, it is not rare. If you want out of the ordinary linebred fish, you need a breeder. I can't help with stores as far away as we are from each other, but odds are the fish farms will sell the same guppies here as there. Guppies are mass production fish.
If you really like to keep rare guppy sports, it would be better to find a private breeder instead of a store. Unless a store is specialized in rare guppies. Or try an online store that sells rare guppy sports.
There are many rare guppy sports that you won't find at an average lfs. But rare means mostly also higher prices... Keep that in mind...
If you start with a wild guppy, each mutant fish was a 'sport'. When people liked the "sport". they would select it for a breedig program to try to get it to breed true to form. If the strains are set by selective inbreeding, you get a "strain", "type" or "breed". All are names for human modified forms of guppies.
The actual natural fish is half the size of the sports, with a small tail and a few body spots.
You can look at local small business fish stores. I have this store that sometimes has fully flushed out strains such as red dragon dumbo, full gold, and blue lyretail. Near it, the frmales are separated out and you can get the female that matches with the male. After a few batches of fry, they will start to have pure babies