I can't buy dotmatrix ribbons in Sweden for my Panasonic KX-P2023 from 1996 and have to order them from England.
I have read that people take out the used ribbons of the cartridge and soak them in ink and then remove excess ink with blotting paper. Sounds messy and in need of too much ink.
I have used a simpler technique with applying the ink with a simple water color brush on the ribbon without opening the cassette.
One cassette took 30 minutes to reink and I put on ink then moved the ribbon two cm forward.
The drawback of this technique is that it gets easy excess of ink.
But I waited 3 weeks for the ink to be absorbed and get a rather dry surface. Almost every day I moved the ribbon forward. Printing worked very well but the first 5 pages of print had a little too much of ink. The ink I used was not printer ink but instead an old bottle of Pelikan ink for stamps which was dark blue. As I got it free from my father it did not cost me anything.
For those in Europe that wants to order dotmatrix ribbon casettes I recommend Stinkyink.com.