To book onsite have a look at all the european websites (disneylandparis.fr .co.uk .it .nl ....). They have different offers and you can book with any of them. And they often have better offers than the US website.
To save, you can also book an hotel offsite. The first stops of the RER are all convenient. Some of the hotels in Val d'Europe hotels are even in walking distance to the parks. I've stayed in Bussy Saint George and Torcy and it takes just 10-15 minutes by RER, not much more it would take to the shuttle bus to go to one of the onsite hotels.
However, the DLP resort is quite pretty and all walkable. Staying onsite for a once in a lifetime trip can be worth it. Just manage expectations: hotels and level of service are different in Europe from the US. Disney Tourist Blog has a lot of posts for DLP planning and also one explaining how it is different from the US parks and what to expect. In particular, this post:
http://www.disneytouristblog.com/should-you-visit-disneyland-paris/
is a good read if you're still deciding how much time to budget for DLP.