One way would be to propagate them by cuttings - usually commercially grown varieties, particularly hybrid varieties, of any plants, are propagated by cuttings, as they may not 'come true' from seed, ie there may be some genetic variation between the seedling and the parents.
Whereas with a cutting you are getting an exact genetic match each time.
what the grower does is spray them with 'ethylene' (C2H4)which is a natural ripening hormone found in plants and fruit crops etc. It promotes/acclerates ripening. In effect the grapes are sprayed with this and are forced to grow and ripen faster. they are forced and therefore have not enough time to develop seeds. But if you look closely sometimes, you will find find immature seeds inside the grape, even though c2h4 has been applied.
Cool eh. Dont worry ethylene is totally safe. They use a manufactured form, which is identical to the real thing. Bananas arrive here totally unripe and green. They are put into special rooms and given ethylene also to ripen them quickly.
There was a case a few years ago fyffes Bananas applied pay 10% corporation tax (manufacturing rate) as they reckoned that this treatment with C2H4 constituted processing or manufacturing making them Irish bananas!! I think they were successful .. does anyone know differently?