Yup and it's expensive.
I just go to the local fabric shop and purchase a couple of yards of Dacron fibre about a centimeter thick and cut it up into appropriate lengths.
I just use a good pair of scissors and cut a long centimeter square strip the width of the material, then cut it to the length needed, which is just slightly compressed against the powder charge when the bullet is seated.
Someone came up with the idea that the Dacron fibre add weight and had to be factored into the load. Well OK, if you believe in the tooth fairy.
The Dacron filler, just like Puff lon or Grex weighs less than a grain.
No, it doesn't leave residue in the barrel either.
The cost of a couple of meters of Dacron fibre, which is used for pillow stuffing and liners, is about a third of the bottled product and a few meters will do hundreds, if not thousands of cases. Depending on the case of course.
When I was experimenting with 2400 powder in 30-06 and a couple of other cases, I used Dacron fibre to fill the air space, so I would get consistent ignition and not a detonation.
I still use it for cartridges like 45 Colt, where the case, which was originally for a full charge of black powder is now being used for a relatively small amount of flake, smokeless powder. Not so much for safety as to keep the burn more consistent and groups tighter.