It is inevitable, at some point your marker is going to run dry. Hopefully that is due to all the amazing art people have you doing or you are doing for yourself. When that day comes, you need to go out and resupply.
One of the reasons that we decided on using Coptic markers is that they are refillable and the tips are replaceable. Fortunately refill ink is a little cheaper per pack than the market was, and it will refill a marker at least 4 times. So a cheaper market is easier to start with, it will be much more expensive in the long run.
The same goes for the liner markers. Sadly you can’t just pour in the ink, but the refill cartridges are cheap. Though really we trade out tips far more often than we need new ink just mangling them accidentally (especially the smallest ones)
So here is my latest haul from OtakuFuel. Their shipping is quick. The service is amazing. To top it off they have more in stock (at a great price) than anyone else I have run across. They are not paying anything for this endorsement. I just hope to be able to do business with them for a long time to come. So go check them out and stock up with confidence.