I had the "B1/B2" visa issued a long time ago and it was valid for 10 years: issued in London. Of course, my passport expired during this time so I had to carry the old one with the visa in it when ever I travelled to USA. I now live in BVI and have to renew: the nearest US Embassy/Consulate is Barbados, so I'm gonna have to schlep over there soon.
You can get a lot of details from their web-site (sorry, you'll have to search for it - I cant remember what it was) and book an appointment on-line. Even having booked, you need to get there very early in the morning since I think they use the "block-booking" system, like my dentist used to!!
You cannot get a Visa for any country, as far as I know, in the actual country - it has to be got from the Consulate/Embassy - so Puerto Rico, being a commonwealth of the USA wont work for you as there aint a US em bassy there.