1. Good phrases are:
"I am writing with regard to your advertisement..."
"I am writing in response to your advertisement..."
"I am writing to apply for the position I saw advertised in..."
I good site to see examples of such letter writing is
www.aueb.gr/lessons/english/int_eng3/int_eng3_letters.html+%22i+am+writing+*+*+*+your+a dvertisement&hl=en]this one[/url]
(that's the Google cache - the page doesn't seem to exist any more)
2. D. "Isn't available at the moment" is perfect.
Hope this helps!
[Edit]I edited the URL to eliminate the horizintal scrolling. :D [/edit]