I own an eMate, so I can answer this one.
Any kind of stylus will do. Well, any pointed object would do, but "official" styluses (styli?) whether they're Palm, or from another type of PDA are best to use because they won't damamge your screen.
That being said, even if you don't have the stylus, you can get at 90% of the eMate's functionality. I know this because part of the screen died on mine. I can see everything, but the 'touch sensitive' part isn't working.
This brings me to my next point:
When you get them, find out if they've had the hinge modifaction done. If so, you're laughing. If not, I would reccomend that before you put them into any kind of regular use, you make this modification.
I used my eMate everyday for a year (took class notes on it) and I ignored the warnings of others, and never looked into fixing the hinge. (It's a huge process, that involves taking the eMate *completely* apart).
Now, in addition to fixing the hinge, I have to find a replacement ribbon cable, or find some way to mend the one I have.
In any case, it's a great little machine, and a piece of Apple history to boot.
iMac: 24-inch 2.4Ghz Core 2 Duo
iPhone 4 16Gb
Cube: DVD-500/1.5/120- X.4.8
eMate & Newton 120