You can use the stamp pad felt (for dye inks like the reinkers) to make your own ink pads. Just cut it to size and store it in an airtight container ( I use ziploc bags ). They last for a loooooooong time, but if it dries out, you can spray a little ink refresher on it to get more impressions before having to re-ink it.
The felt can also be washed and used again. Of course there will be staining, but it won't affect your colors.
Sorry, I didn't mean empties, I meant that all of the colors are still available. However if you're making your own colors, then the stamp pad felt that Kim posted above is the material you're looking for.