If you are a D23 member, you can purchase all of the releases in a set at the Disney Store D23 tab. Of course, it is pretty pricey for 10 years worth of DVDs.
I was about to ask the same question. I found a few on ebay,..I regret not getting these when i saw them in stores...there are so many classics that you need to have in your collection.
When the Disney Treasures are first released, Best Buy usually has deals and promos. After that, particularly for out-of-print titles, Amazon.com is your best bet. AND remember, Amazon.com doesn't charge sales tax.