Stock footage search service
I don't know what you mean by human readable search terms, as opposed to keywords or tags?

But for a meta search directory service I would recommend (know of)
http://www.footage.net/
(If anyone knows of any additional search engine stock video directories, I would love to know about it as well)

Otherwise, ya, you would be going through each service looking for footage.
These are the stock services that I am aware of:
http://www.shutterstock.com/video/
http://www.gettyimages.ca/footage/ &lt;- used by major media outlets
http://www.pond5.com/
http://www.alwayshd.com/
https://www.videocopilot.net/products/ &lt;- geared more to adding effects to clips
http://www.footagesearch.com/

And many more... I am sure others can chime in with their favorite sites.