Reddit mentions of Pentel Arts Pocket Brush Refills, Black Ink / Value Set of 12 （With Our Shop Original Product Description）

That’s a really good question! I’m gonna admit I’m not too familiar with the Brush Pen. I did just take a crash course and this is what I’ve found out.

Amazon has a Prime listing for 12 cartridges for about $9

JetPens sells a pack of 4 for about $5. I read the reviews and everyone loves the ink but someone did say this: “If you get a syringe you can refill the cartridges when they are empty with any ink that will flow into the sponge reservoir in the pen, but I've never been able to adequately match the ink in these lil dudes, there's some magic ingredient that makes it amazing so refill packs it is.” I will be making an order to JetPens really soon but it looks like a very solid reputable site to buy from and possibly even ask what ink would work well with this pen. I’ve been browsing r/fountainpens here recently so I know it’s possible to refill cartridges. I just don’t know if fountain pen ink would work. If it does that would be great as it comes in millions of colors.

Did find YouTube link describing how to refill cartridges for that pen. He uses a waterproof ink.

So basically it’s a matter of preference. If you really love the ink it comes with you may want to stick with cartridges which can be found cheaper than what you’re currently using. If any ink will do and an entire bottle of ink doesn’t scare you it looks like all you need is a syringe and a steady hand.

Hope that helps and let us know what works!

Edit: all this info is for the Pocket Brush pen not the color brush pen. Not sure which one you have.

Edit 2: comment got stuck hung up somewhere in invisibility idk