I'm bumping this thread to highlight a few things Chelmno-related
Firstly, in addition to the Lodz ghetto Jewish council records, a very big chunk of the records of the Nazi Lodz ghetto administration (Gettoverwaltung Litzmannstadt) have been digitised and are online at the Polish digital archive. A total of 1.3 million pages. The GVL records contain a number of important sources on Chelmno.
Secondly, Yad Vashem has digitised and put online a file of Goering's Forschungsstelle A Litzmannstadt with over 300 pages of phonetapped conversations summarised into reports, all concerning the Lodz ghetto administration and to a lesser extent, the Lodz Jewish council.
Thirdly, these sources give us Nazi documents regarding cremation at Chelmno, both explicit as well as sources on deliveries of e.g. iron girders to a recipient that is undoubtedly Sonderkommando Kulmhof, which help corroborate witness statements and add interesting detail to our understanding of how cremation developed at Chelmno, from simple pits to grids to improvised crematoria with chimneys.
I've written up the key points in an essay-length blog post, please excuse the anti-Holocaust denial remarks, they shouldn't be too obtrustive for those simply interested in the history
http://holocaustcontroversies.blogspot. ... .html#more
The blog post and its footnotes also give links to the digitised collections mentioned above.
Hat-tip to spottedcow for his book on Chelmno, which was very helpful writing up the essay.