I'm dabbling with ideas and have no experience in this area.

I have a rainfly from an older double wall tent that I got from a garage sale. The tent is still useful but the fly seems small compared to contemporary designs. I would like to build a new rain fly.

I'd like to use this opportunity to try using silnylon.

First is this a good material for a rainfly?
Where do you get the stuff?

I think sil would be a great material for a rain fly. Check this thread from earlier this week for a list of sources:
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=17925

This thread on materials sourcing will tell you where to find just about everything you need:

http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/1740/index.html