I will be traveling for 6 days throughout Switzerland and need a rail pass.
Should I buy it before I leave the US or can I get the same pass/price when I arrive in Geneva?
Also I have found several places to buy the rail pass, is there an "official" site?
Yes, you can get a pass at the Geneva airport train station or the main station. www.sbb.ch is as official as you get. They own and operate the trains.
As noted by Sam, you can buy the Swiss Rail Pass when you arrive in Geneve, at the train station in the airport. The official site is the Swiss Rail website - www.sbb.ch/en. where you can find details and prices for the various Swiss railpasses. Many other agencies also sell these passes, including Rail Europe, which is generally a bit more expensive than buying directly through SBB. Currently the 4-day Swiss Travel Pass is 251 SF ($263 U.S.) through SBB as opposed to $302 per person through Rail Europe.
There are several different types of Swiss railpasses. I suggest reading through the various options on our Swiss railpass page to determine which pass is best for your trip: www.ricksteves.com/travel-tips/transportation/trains/switzerland-rail-passes