I got mine (model 200) from B&H, great price, which I would think would be hard for the e-bayers to beat, and B&H will still be there for a lot of tomorrows.
I purchased my slingshot from this seller as well. He was great - I bought the 200 and then decided that was too big. I contacted him about getting the 100 instead and he shipped it out right away. I had to pay for return shipping on the 200 but that was fine. Still saved money compared to getting it from a brick-and-mortar shop.
Take a look at cambags.com. You'll need to click on the "slingbags" icon at the top, then navigate the left pane of the window to find the Slingshot 100. They show a D70 config that may show you how much space is available.
I just purchased a Lowepro Fastpack 350 after reading and reading online. i have a 17" laptop that I wanted to be able to carry on the airplane with me and I really liked the idea of a backpack over a single shoulder bag. I ended up buying on ebay because the no tax and free shipping I couldn't find a cheaper price.
I read lots and lots of great reviews on the SS so I hope you enjoy!