We will be doing a self drive tour and don't want to lock ourselves in to a very tight schedule with a lot of advance bookings.. Can we breeze into towns like Obidos and Tomar and find local private guides to show us around the historic sites?

Obidos is a small town that has become quite filled with tourists. You do not need a guide to walk around this little town. Two hours is all you need to see it all.

The Michelin Green Guides provide good information about the sites, especially if they are considered 3 star sites.

Ditto to what HappyTrvlr has told you about Obidos.

For a tour of Tomar with visit to the Convento de Cristo Templars' Castle and to the Tomar synagogue and a walk around town, I've used the services of the local company, Caminhos da Historia, for friends. The guide met them at the site, guided them through and followed by the walking tour of the town. On my own visit, there was only 1 audio guide available which my friend and I had to share, so I found a local, licensed professional guide for these friends.
The web site isn't in English but my communications with them via email were all in English.

www.caminhosdahistoria.com.