Valve certainly have banked on this particular idea, with the concept of the Steam Machine moving along at a [for Valve] rather brisk pace. You might be able to pick one up as soon as November, 2015.
The Valve Store page now includes listings for Steam Machines that ought to debut then. Some of the prices are pretty reasonable [$400 or so] while others are downright crazy [$5000].
Given these prices, though? You're probably better off just custom-buying your own machine.
Feel free to follow along past the break for the list of potential third party vendors:
Alienware - $479+
Alternate - $1099 - $1999
Asus - $699+
Digital Storm - $699
Falcon Northwest - $1999 - $4999 [!]Gigabyte - $599
iBuyPower - $459
Maingear - $849
Material.net - $899
Next - $799 - $1299
Origin - $899 - $4999 [!]Scan - $999 - $1299
Syber - $499 - $1399
Webhallen - $949
Zotac - $999