These are some brush pens with piston refilling mechanism that I bought from Straits Art recently. Each is S$9.50 (US $7) so that's really affordable.

I can't see any colouration with the hair so I guess they are synthetic.

These brush pens are rather generic and they have no brand. You can probably find them on eBay or Amazon.

They are all made with plastic. Build quality is so so. They don't look that good.

The one with the clear demonstrator body can be dismantled to a certain extent.

I don't recommend using heavily pigmented inks as they will clog the brush pens and since you can't dismantle the front, clogging will make the pens unusable.

Hair of the demonstrator brush pen is about 1cm which is similar to the Pentel Pocket Brush Pen. The demonstrator brush pen has the advantage of being easy to refill, and is cheaper also than the Pentel.

The black brush pens have 2cm and 2.5cm long hair.

The brush pens with shorter hair have better ink flow. The strokes in orange are from the 1cm hair and the black from the 2cm hair.

Actually the ink flow is so good that you can expect a wet surface and a much longer time for the ink to dry completely.

You can create splatters easily by running the hair against a clip or card.

It's not surprising ink flow is not able to catch up with the 2.5cm hair because that's huge and uses so much ink.

The hair has some spring to them and can return to shape. However for the large hair brush, you have to point it yourself if you want a sharp point.

The smaller brush pen is better for drawing as it's easier to control the point.

The larger brush pens are better for painting. They are great for tonal studies. The inks I use are Noodlers Bulletproof Black and Lexington Gray. They both flow well and are waterproof when dry.

Having the piston makes it easy to refill these brush pens.

I don't recommend using these as waterbrushes. With waterbrushes, you can squeeze the body quickly to get more water out for mixing. With these brush pens, you have to turn the piston so it's inconvenient.

I've actually bought several piston brush pens years ago. Those were natural hair brush pens and the hair fell out rather quickly. And I remember the hair being quite soft and floppy so they are not great for drawing.

The brush pens shown above seem to have better binding for the hair. Hopefully they will last.

See if you can find them on eBay or Amazon (US | CA | UK | DE | FR | ES | IT | JP)