Where can I buy a copy?

Right now, it's pretty hard to get a copy in the US. There is a Fox-Lorber/Wellspring version, but it's been long out-of-print. People are selling them on Ebay and Half.com though. A couple months ago, this sold on Ebay for 150-200 dollars. But in the past couple weeks, it's gone for about $40-50. The video quality is not the best, and it's not anamorphic--likely a laserdisc port.

I have been loking for this dvd and have finally bought it in amazon.fr at 20,99 euros (25 dollars). It is a dvd r-2, in french with no english subtitles, so you would need a multi-region dvd player and be able to understand french.

I have the Fox Lorber DVD, and it is...adequate at best. However, Rialto Pictures has it listed as a "Future Release" here:

http://www.rialtopictures.com/catalog.html

Given that most of the titles in their catalogue have been released on DVD under the Criterion imprimatur, I would expect this one to get that noble outfit's treatment (anamorphic, restored and loaded with extras, one hopes) within the next year or so. Take heart!

Edit: Here's what I have in an email from Rialto: "We plan to do a theatrical re-issue of Marienbad, hopefully sometime in 2006. The DVD would follow at least nine months after New York opening date." Accordingly it looks as though 2007 will be the year.

you can get a region 2 English subtitled version from Amazon.co.uk among variuos other U.K. websites.

Just got a copy of the R2 Optimum World version from Amazon UK: good image and sound quality, as well as good extras, including Resnais' earlier 'Toute la memoire du monde'.