Where can I purchase Paris Metro and RER tickets in the USA (Paris Visite)

The Paris Metro is a subway. You just buy your ticket and get on. It's the underground local. It's like the subway in L.A. or BART in the S.F. Bay Area. There's nothing "advance" about it. I haven't riden RER.

You can also buy a carnet of 10 tickets that will cost less than purchased seperately. I'd gotten the impression that the Metro is more like the SF Muni subway and the the RER is more like BART, but, again, that's my impression before actually seeing them yet.

The Metro is a mostly underground subway, like BART. It only covers Paris itself. The RER covers a limited part of Paris and a large area around Paris. In Paris, the RER is underground. Outside Paris, it is above ground.
As Larry posted above, you don't need to buy tickets in advance, and I've never heard of doing that. If you plan to ride at least 10 times, a carnet is the best way to go, you get a discount. There are cards you can get for longer periods of time, but it's a little more complicated and you need to provide a passport size photo.

If you are staying for about a week or more get a Carte Orange. It's a metro pass, unlimited. Good from, I think, Mon to Mon. Available in the metro stations. You will need a picture but it doesn't have to be passport standard. We took pictures outside the station and then crossed the street to the Kodak shop where the young man inside was very helpful, printed,cropped them to size and Voila, we were good for the week. It cost us E1 each for pic's. If you use it 2x's/day, it's well worth it. It was our only transportaion for 5 days and it was still a deal. I'm not positive what we paid, I think it was E16.

Do not buy them in advance. It is very easy to purchase at the metro stops. Also, even though you did not mention the museum pass, do not purchase that in advance either. The companies that sell it in advance charge an outrageous amount to ship it to you.

The 2 times I have tried to take the RER it was in strike. I would buy tickets at the station. Good luck