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.