Agree with edrummer. go for a 16" or more if your two existing toms are not floor toms because this gives you a lower,deeper tone, and therefore there will be a more significant difference when you hit a 12" and a 16" as compared to if you get a 14". Then again, if it is merely for practice or you have space constraints, you could always choose otherwise.
And yup classifieds like the drums buy and sell section have good deals coming up sometimes. better to be patient and get a good deal haha, rather than going to some commercialised music store and perhaps overpaying. You might want to get a tom that matches your set though.
Good luck choosing a tom! =)