Welcome aboard! :D
There is no training material in the public domain (though you can learn a lot from this site!). Ascential does offer web-based as well as instructor-led and site-based training, details of which can be obtained from them (or their web site).
There is no demonstration version of DataStage. If you can make a sufficiently strong business case, with a reasonable prospect of eventual purchase, you may be able to obtain an evaluation copy (there may be a fee involved if this does not result in a purchase). Again, contact Ascential.
Ascential keeps a tight rein on their training material, which is somewhat understandable given how rapidly the technology is advancing. I used to create curriculum when I was an employee of Ascential's predecessor companies but these materials, of course, remain the property (intellectual and otherwise) of the company.
Some contractors do offer a training/mentoring service as part of their offerings, which might include their own notes on best practice and so on, but none that I know of offers training materials per se.
As download areas become more used both here and at
Ascential Developer Net it may be the case that some "tutorial" material becomes available there. Certainly, Ascential web casts can be downloaded from their site.
And, of course, the DataStage software itself ships with a tutorial. The tutorial has its own manual in the documentation set.