I can't answer your question about purchasing Disney Dollars online. My gut says you can't but I'm not 100% sure.
And yes, they are accepted for payment of room charges upon checkout. I did this just last week at Pop. The CM basically said they are the same as 'cash'.
Hope that helps!
I thought you could still get them from Disney shopping online.
I have not had the opportunity to do so, but I was sure I was it last time I was browsing the web site.