No official sellers in US exist. Maybe somebody can arrange shipment, but you should expect problems with mobile network frequencies. The Fairphone 2 cannot use 4G in the US because it does not support the frequency bands used for 4G in your country. 3G availability will be limited too, of the three frequency bands used in the US only the 1,900MHz one is supported (note that Fairphone 1 lacks support of this band). At least Verizon and Sprint appear to be phasing out 1,900MHz 3G in favour of unsupported 1,900MHz 4G, and AT&T appears to offer 1,900MHz services only “in some areas”. T-Mobile on the other hand appears to be moving all 3G services to the 1,900MHz band because their 4G network is on a different band, so that would be your best bet if you live in an area with T-Mobile coverage. (http://www.fiercewireless.com/wireless/t-mobile-shutting-off-hspa-service-its-aws-spectrum-market-by-market)
On the other hand, 2G is supported on all common frequency bands, but having to use a modern smartphone with a 2G network is quite sad. Also, I have seen a few reports from 2G-users here that their carrier was phasing out 2G and left the phone unable to connect. Not sure though if that was about the USA or another place outside of Europe.