Sites off the top of my head - Craigslist, Ebay, CycleTrader, Ben's Ohio Riders, bike forums.   
As far as the bikes being identical... 900 Custom is a nice, smaller, cheaper cruiser, and it shares some lines with the Mean Streaks, but that's about where the similarities end.  Here is a pretty good thread -    
http://www.kawasakimotorcycle.org/forum/kawasaki-cruisers/60706-help-me-out-vulcan-900c-vs-mean-streak.html   
Oh, and everyone has different opinions on what the first thing should be that you buy for a bike.  Hands down, an aftermarket seat.  I have a Corbin, and it makes a HUGE difference on longer rides.