There are lots of seed sellers on the Internet, but few of them actually will ship to the US. I've found two reliable ones that offer a wide selection of strains from many different breeders and will ship to you from the UK in stealth packaging without delay:
Seeds are legal to possess in and export from the United Kingdom. If you order seeds from a distributor in either The Netherlands or Spain, where it's tolerated to grow, but not to ship to the US, they will ship them via Switzerland or the UK, if at all. That can take more than a month, sometimes to receive the shipment. There are companies in Canada that will ship to the US, but the distributors in the UK offer most of those same brands, as well as seeds from breeders in Humboldt County, California.
http://www.worldwide-marijuana-seeds.com
http://www.sensibleseeds.com