The Dell Outlet is a good source, also the Lenovo Outlet. I'm a Thinkpad user and fan, and while others have been catching up I still think they're the best.
I'm typing this from a Thinkpad 13 I got for just under $400 a few months ago. It's a Core 3 with 8GB and a 128GB flash drive, Windows 10 Pro. It's about equivalent to a Macbook Air 13, a bit thicker and not as sleek, but a better keyboard and screen.
I regularly see the top of the line T Series (the one you want) for half price or less.
Keep checking for deals as they change every day! Sometimes really cheap pops up among the usual. Mine, for example, was $1-200 less that the usual outlet price at the time, and $3-400 cheaper than full price.
They have desktops too.
BTW, you don't need Windows 10 Pro. These days the Pro version just has extra tools for managing fleets of computers -- nothing you need for just one, or a few.