I feel your pain =\ last year i was moving, and this driving my disco from cleveland, oh to boise, id. Somewhere in the middle of Nebraska the alternator decided to start destroying itself. So my dad and i went with the buy a couple of batteries and a charger method.
It worked out well for us. we found that a battrey would last for basically a full tank of gas (~3hrs iirc). so we stopped for gas, changed out the battery and kept going. but... the amount of time you'll have on a battery depends on how far gone the alternator is. I believe mine was producing a little bit, so it would feed the electrical system a bit, but not enough to actually charge the battery.
If you do decide to buy some batteries and drive first i'd follow RoverChic's advice. Run nothing that isn't absolutly neccessary: no radio, no ac, no fans, no lights (thus drive during the day if possible), etc. Also I found that walmart is a great place to get the spare batteries since they took the battery back with no questions asked.
Good luck!
Sorry, but I had too! 
