Glass files have made a great entering into the modeling world, especially in the Gundam community.
They are less aggressive than metal files and allow for much higher grits than these, and they stand in a perfect condition much longer than traditional sanding sticks or sponges.
See pictures for reference of grit and size.
Note: it is important to keep the files clean to obtain good results. If you allow dust to accumulate in the crevices, they will lose their "bite". To clean them, use tap water.
We have to prepare each shipment individually as they are ordered, so allow it in 3-4 weeks to arrive in the USA/Canada (to be safe) and 3-6 weeks rest of the world, though they are usually delivered much faster.