I usually buy my tickets from Trainline, but I believe they are now charging a small fee for tickets purchased from IP adresses from the States.
I think you may be able to get around being routed to Rail Europe on the sncf/oui site by choosing UK as your country. If not, choose France but then just click on the UK flag to get the page in English.
As Laura mentioned, SNCF does not sell tickets for the Paris suburban trains known as the RER. So far as I have ever known (anyone can correct me if I have it wrong), those tickets can only be bought once you are here in the Paris region.
TER trains are local trains all over the country and should be able to be purchased through SNCF/oui.
For help on all things trains in France, check out the excellent “Man in Seat 61” website and click on the France page.