If you're reasonably handy with tools you should be able to knock one up in no time.
By way of example.... 1. Get a bike. 2. Make a frame that will suspend the back wheel off the ground (you can buy these also if you want). 3. Take the tyre off the back wheel. 4. Find an old generator/alternator from a car. 5. Hook the terminals of the gen/alt to a car battery. 6. Run a belt around the bike rim and around the pulley on the front of the gen/alt. 7. Peddle like there is no tomorrow, which of course would be true if you needed to make power this way :-)
You might also want an inverter to convert the 12 VDC to an AC supply of your choice.
Edit: Here's a plan, not that I recommend, it was just first on google (keywords: bicycle generator plans)
http://www.econvergence.net/electro.htm