If you can hold on until September 24th, there'll be a Bike Sale (Velo Boerse/Börse) at Bürkliplatz in Zurich, between 9am and 3pm. Get down early and check what's on offer - as usual the good stuff goes early. Bring a fist full of cash, and don't expect to bargain, it's usually not done on these occassions (but you can try if the bike's a ghastly colour).
Usually there're two prices on the tag: one the sellers price and then the commission price from the organisers - something like 10% or so. You simply wheel the chosen bike from its place to the stall where the organisers will 'clear' it and there you pay. And then you cycle home

(bring a bike lock before you go if you intend to do further shopping on the way back!)
If you can't wait, there'll be an earlier Börse in Winterthur, at the Neumarkt, on September 16th.
Happy days. Oh, by the way, get one with mud-guards. It rains a lot here...