A friend of mine was going to research the chemical in the pads and see if it would be possible to resoak the pads as they are the expensive part of the Thermacell. He has not reporte anything back so far.
The big auction place does always have bigger cartridges for cheap but they use an adapter, which is usually included and you have to leave the cover off as the cartridge is longer.
I have a thermacell in my pack but I rarely use it. Permanone on my clothes,face mask, gloves etc seems to do the trick and keep the bugs away. If the buzzing gets to bothering me, I will fire up the Thermacell for about 30 minutes and shut it back off. I can get a lot of time from a pad and cartridge.