Hi,
It won't be an enormous improvement, buy you will probably have more luck if you look very specifically for what are called "stepper motors". What you describe as "small DC hobby motors" makes me think of the tiny things found in toys, so no wonder you can't get much out of them.
"Starting from first principles", (this is my revenge: I've had plenty of professors announce this phrase to me) motors typicall consist of a rotating component and stationary component. Small motors usually have a rotor with many windings of wire on them and the current flows through brushes. The housing has a pair of magnets, maybe one pole on one side, one pole on the other, or maybe four poles in some cases.
Producing the voltage is a matter of capturing the magnetic flux with loops of wire - the more loops of wire you have or the stronger your magnetic field, the more voltage that can be produced.
The current, however, is on the other side of Ohm's law. If you have more windings of wire on the rotor, then the wire used is very thin, and there's a longer run of it. You get more voltage from doing so, but the resistance of the winding prevents much current from flowing. Catch 22.
Can you fix this? Not really. Not with "DC hobby motors", anyway.
This is why you will have more luck with stepper motors. The design of a stepper reverses the order of magnets and wire. It is often what we here are doing, when converting induction motors (yet another type) for use as generators. You can find stepper motors with thicker wire because there's more room for it on the outside, and small permanent magnets work efficiently in the core.
That said, even if you can coax a few watts out of one, you are doing well. They also have a tendency to "cog", because the attraction of the magnets in the rotor holds them in place before they can be turned to the next "tooth".
Try reading a bit of this for more suggestions:
http://www.c-realevents.demon.co.uk/steppers/stepmotor.htm
Computer recycling is common nowadays. Printers and photocopiers by the thousands are thrown on the heap with them. Finding motors out of them shouldn't be hard. Little inkjet printers may not be ideal, and I've noticed that HP laser printers have a mix of tiny steppers and circuit-board-mounted motors that probably won't be useful to you. Xerox and Toshiba commercial photocopiers... now that's the jackpot!
Strangely, the companies that conduct the recycling may not want to give up their prizes. Not that I'm suggesting you snag a few that were dumped at their back door. No no...
Nobody EVER does that!
