Buy 'em like you buy anything else..whip out the credit card and go to town. There's lots of sites that sell seeds, and it isn't so much getting the seeds that is a problem...it's getting GOOD seeds that produce quality smoke.
Currently I'm growing some stuff I bought from
marijuana-seeds.nl White Widow and Northern lights. PLanted 5 seeds so far and got three females. Haven't finished any of their seeds, so I can't say anything yet about the finished product, but their prices can't be beat, so it's worth a gamble for me, and the shipping charges were cheap. I would recommend them for any first time seed buyer.