I just came across this post and thought I’d make another suggestion, seeing as how there was only one response. You could try Panoskies, www.panoskies.com they make hires unclamped HDRI Skies for arch viz, vfx etc. Unclamped means that the full intensity and true color of the sun was captured, so you’ll get very accurate and realistic lighting.
I should declare my interest here in that this is my website. The HDRI’s are not free but bear in mind there is a certain amount of work involved in creating good HDRI’s. There is also a free sample for anyone to try, you can download it here,
All of the HDRI’s from Panoskies were tested in several render engines including Cycles, so they’ll work fine. Just make sure to use Importance sampling and set the map resolution to at least 2048 for good results.
Also remember that Cycles doesn’t tone map it’s renders so if you use any HDRI as a light source you’ll get a darker more contrasty result than if you had used a different render engine, such as Corona for example. You’ll need to tone map your Cycles renders in the compositor with the Tonemap node to get a correct looking result.