One may simply empty their sander bag and have all the very fine wood flour for epoxy filler they might ever need. If one is like me, a sander bag gets in the way and gets lost amongst countless other unused items, somewhere in the jungle known as the garage. So one like me must improvise...
A coworker provided a grocery sack with some red cedar sawdust which I sifted by hand over a #120 stainless mesh and got a pint of very fine wood flour. Ground the remaining in my blender and got another pint. That's all the sifting I care to do for now.
If you ever do a stitch & glue, you'll know what very fine flour is if you try to put anything bigger with epoxy into a dental syringe and expect it to come out.
Personally, I like the look of cedar flour as it darkens red when mixed with epoxy and goes well with mahogany while I'm sure some prefer the honey color of the standard wood flour provided by boat kit companies. Whatever one likes.
Another thing I literally have barrels of is microcrystalline cellulose. It's a very fine white powder used as a filler in medicine pills. If anyone reading this has a need for some white epoxy filler give me a shout. I'll sell you all I can stuff in a large priority mailing box for $10 plus shipping.