You can buy very similar brand new, they are based on old BMWs (very much like those in the film in your sig)
http://www.imz-ural.com (although I wouldn't pay anywhere near the price they are asking!)
http://www.dnepr.ru/motogal.htm (same bikes)
Also check out:
http://www.enfieldmotorcycles.com/
And
http://www.triumph.co.uk/usa/786.aspx
Try
www.cycletrader.com and you can search by make, model, year, zip code etc. Also, there is ebay.
What about something more upto date, but considered a modern classic, like a good condition Suzuki GS, Kawa KZ, Yamaha XJ ...all available for $1k-2k except for the Kawa Z1-900 which has 'cult' status...
In the 70s Kawasaki/Suzuki also made some mad-as-hell two stroke triple cyl bikes that'll make a man of you if they don't kill you first! KAwa KH and Suzuki GT are the models...
Food for thought - the 70s/80s Jap bikes I just listed will handle and stop MUCH better than any of the 50s/60s/70s European bikes...and be tons more reliable...
You can pick something like a '97 B1200 Bandit up for $2.5K-3k, a decent condition classic brit/german bike can cost twice as much...
What do you think of these ideas?
If we can establish what you want, how much you want to spend, how often you want to ride it, etc, then it will be a lot easier to point you in the right direction