I just found the below link, which has a group of incredible sky panorama's, that work VERY well in BagginsBill's Env Sphere, or I suppose any dome.  (for sphere, I doubled the height -100px, so if it was 2000px high, I added 1900px to the bottom)

In any case, as I quote the creator from his Blender Artists post:

Hi folks,

I thought it was time to give something back to the community again, so I've released a new high resolution texture pack for Blender 3d.....there are 19 textures in all, ranging from 10k in size to 16k. These are much higher resolution than any textures I've released before, and should be suitable for full HD rendering.

....

Released under an 'I don't give an damn' license. Meaning you can use them for anything you want, including commercial work. The only thing I ask is that you don't sell them. That's it. Have fun, and make great art!

LINK (note, link in description of how-to video)