Hi Sami,
If you can wait a couple of weeks, the lost property office is auctioning off unclaimed bikes (*forgotten* on trains etc) on December 6th, starting 3 pm.
The address is Bahrenfelder Str. 254-260 (but the entrance is actually Gausstr. Haus 2), just around the corner from Altona Station. This is probably the cheapest legal way to acquire a bike!
Any idea when the next one of these is?
Totally forgot about it this time
Still quite keen on getting myself some wheels, especially for when spring eventually kicks in...