I'd just purchase a prepaid Suica card at HND at the same time I exchanged the Rail Pass. Once it has some money stored, you are good to go for subways and locals. You can even use it for convenience store purchases. The only thing is that you will need some cash to add more funds to the card if it runs low.
I always buy the Rail Pass from authorized dealers and have it send to my home via second day delivery. I've seen some unauthorized dealers offering somewhat lower rates but prefer to know who I am dealing with.
http://www.japanrailpass.net/en/
http://www.jreast.co.jp/e/pass/suica.html