How to approach a casting agency?

I'm currently in post-production on my new film and I require some people to play:
i) a doctor
ii) The Pope
iii) an old father

I'm casting actors I know as the other parts, but the parts mentioned above are different in that they all require middle-to-old aged men. I don't have access to any of those people so I'm considering asking a casting agency.

but I have a problem - how exactly do I approach these agencies? Just call up and go "Hi, I need some old men"?!?!?

any help would be great
Thanks

LOL

Do you get shooting people?
If not get it here> http://www.hostboard.com/ubb/smile.gif

I often see requests for old folks on here, I don't know if they're successful but it's worth a try.