Hi!
Having just started here as well, I can share my experience with you. I just picked up a very nice and completely stock CB550 from the original owner with a mere 5000 miles on it. The bike is so darn clean and original that I'm now second guessing any aspirations I had to go cafe style. I really think I got lucky - but also some leg work and research went into my purchase.
I started with
http://newyork.craigslist.org or New Haven area etc. and search every day for "1970", "1927"... - "1977" which is the date of the bike I wanted. There were a few there that would suit me, but most were in rough shape and I really wanted to start with something solid. eBay was where I found my bike and it was located in NJ, about an hour from my home. I decided this was the bike I wanted and had a very reasonable/possible high late bid that in the end won me the auction over some who didn't want to pay or maybe those who were just looking for a cheap bike. I've also found a dealer in corning NY -
http://www.stonebridgecars.com/motorcycles.html who seem to have a '76 cb550 in stock that I was considering. Don't forget the forum here too!
I also decided on the 550 because I actually want to ride the bike. The 3/450's seemed a little small for my tastes while the 750 would be too much. Kinda like the porage in Goldie locks, the 550 was just right for me.

Good Luck!
Ps. message me if you're interested in online stores and places to buy parts. There are tons of links on this forum but I've also got a file of bookmarks I've been keeping of places for parts and upgrades etc. Are you looking for something like oldbikebarn.com ?