The only place that I've seen official replacement 770 styli available is at the Tessco website for $0.75, but unfortunately Tessco only accept orders for Nokia parts of $125 or more

You could try contacting your local NSC to see if they could order 1 for you, however, I think you would end up paying considerably more than $0.75.

Maybe someone here on the board has a spare that they wouldn't mind parting with, the only other alternative would be to buy a stylus from a third party manufacturer, the downside to this would be that you probably wouldn't be able to stow it in the 770's stylus holder.