Where is the best place to buy a Paris Museum pass? We are in St. Germain.

Musée National Eugène Delacroix, 6 Rue de Furstenberg, 75006 Paris, France
... which is merely 250m (3 min walk) from Église de Saint Germain des Prés.
The museum opens today, Thursday at 9:30am.

Charles De Gaulle was incredibly easy for us.

The Cluny (MIddle Ages) Museum is not far and shouldn't have lines.

No secrets, just check the PMP website. The "Buy" link at the top right takes you to various stores and kiosks where you can get one. You can also get one at any attraction the pass covers. If you're near the Delacroix Museum (6, rue de Furstenberg, 6the arrondissement) you might try there. You don't have to go into the museum to buy the pass, and you shouldn't put in the start date till you're ready to enter the first attraction where you're going to use it, because that starts the clock.

FNAC (book/music/technology store chain) sells them with no extra mark-up, if one is near where you are staying.

Depending on where exactly in the 6th you are, I would suggest the Cluny also, and the Pantheon, if closer to the 5th, if closer to 7th, theres Invaldis Army Museum, the Rodin, or Napoleons Tomb.

Right across the street from St. Chapelle, at 5 Boulevard du Palais, is a Service store that sells museum passes and is not ever crowded. It is between the two bistros, can't miss it.

The Cluny. (Musée du Moyen Age).

There's a TI kiosk in the square in front of Notre Dame. There's rarely a line of more than 1. And you can pick up brochures and get any help you need from them too.

Also, at the ticket counter outside Musee Orsay, there is a place to buy the museum pass.

Actually the Louvre is fine. Use any security entrance other than the Pyramid and then once under the pyramid just buy it from one of the clerks. Just if none of the others is convenient. Cluny and Delacroix are both good and the Roding is often not very crowded.

Last year I bought our passes in the Carrousel du Louvre. Quick, and we had a short walk to the underground Louvre entrance.