FTB, it is ok to practice spawning with a real breeding pair! Most first spawns in general are unsucessful, and it doesn't mean you can't just try again after another three weeks of conditioning.
Breeding petstore VTs would be like taking two extremely poorly bred dogs of different breeds, older dogs that you know nothing about the lineage of, and then trying to find homes for all of the puppies in a country like the USA where we're still euthanizing some 6-10 million pets every year due to a lack of homes.
Please remember that essentially all males in pet stores are at or past their breeding prime. This means you have a very high likelihood of deformed fry. Also remember that you know nothing about the lineage of these bettas, though you can assume it is poor since all of Petco and Petsmart's VTs come from one mass breeder - this again means a higher chance of weak or deformed fry. The two bettas you want to breed are completely different color types, which could and probably will throw unattractive, undesirable fry. And again - no one in their right mind is going to pay shipping for VTs when they are so cheap and allready overpopulated; even if you can find a store to take them, what makes you think they won't end up like the rest of the pet store VTs? Half will die in the store, the rest have a lousy chance of finding a home with someone that truly loves and respects the species.
On the other hand, if you just have a little patience and breed a quality breeding pair of PK, HM, SD, or another desired tail type, you know that the fish will be young and healthy because they are straight from a reputable breeder. You can select a pair that is likely to produce quality, nicely colored offspring by buying a pair selected by the breeder based on a good, matching phenotype. And people will GLADLY pay shipping to buy these bettas, so you will not struggle to find quality homes locally or worse still - have to dump them on a pet store. And did I mention you'll actually make some money? You'll probably have to give away VTs, but you can actually sell other tail types, which is a huge plus for you since buying all of the supplies for breeding is going to cost a LOT of money.
Which brings me to my last question... are you even prepared to spawn? As Jaded said, you need to have a ton of jars for the fry as they grow, which costs a pretty penny. You'll also need a spawn tank with plenty of plants and hides, a heater, etc., a grow-out tank, live food for the babies, foods for conditioning the parents, medications in case anything gets sick, etc. Ask any breeder on here; breeding bettas is a serious commitment and a great financial endeavor, not something to take lightly.
You are dealing with the lives of living things here... is it really such a good idea to breed your VTs "just for practice?"