Normally you get the SSD adapter when you order Apple's 512GB SSD drive as an option for your brand new Mac Pro. But you can also find it on eBay. This seller, for example, is selling the adapter only.
Just ordered Intel 520 240GB from Newegg (currently priced at $229, best I found) which includes 2.5 to 3.5 adapter. As a Mac Pro newbie, should that be all I need to install it? The Mac Pro will arrive today...