eh, generally I use boiled/heat-dried cotton from the cosmetics section of my local drug store... I choose that over the medical section because in my mind, the medical swabs are usually treated with god-knows-what, and the cosmetic is free of those chemicals. If you ask for a source, it's my own personal experience, I can *smell* the medical swabs and they smell treated!
I've also used organic, but quite frankly I don't detect enough of a difference to justify the markup.. and any residual chemicals, or contaminates have to be 10x better than the lab-soup that analogues contain! I've used Rayon and it tastes... funky.. to me. and Silica is.. well.. I question the sanity of sucking vapor off of what is essentially fiberglass... call me paranoid but that doesn't sound like a good idea.
In the end these are my thoughts and to each their own, and too that effect I advise you experiment with what's available and see what works for you!