Hello, my friend Mr. Chop made a large sum of skybox textures for use in the Source Engine. Since he is rather lazy, and didn't bother re-uploading his pack since I last downloaded it, I uploaded it to my dropbox with his permission to share with you all.
Here are a few sample skyboxes.
These are completely free, and Mr. Chop/Colter is completely fine with you sticking it in an operation map.
Anyways here is the download (96mb)
Enjoy!
Please forgive me if this is in the wrong section, FMPONE suggested I share with you all as well, so if this needs to be moved please let me know.
Also let me know if the download link goes down as well and I will do my best to maintain it. Have fun!