Transferring to new Theme – How?

Hi all,
I’m looking to develop a new theme for my blog. Can anyone recommend a good free website to get a quality template from that I can modify?

Also, what is the easiest way to test my new theme that I am developing against my current content? I don’t want to take the site down and certainly don’t want it looking unprofeessional for several days whilst I make the changes.

Thanks!
Amnesia.