Sterile Cotton can be used (that's what I'm using right now). The purpose behind Organic Cotton is that it hasn't been bleached white, thus, it's come in contact with fewer to no chemicals. Organic Cotton has a slightly off-white color while Sterile Cotton and Cotton sold from Walmart, Walgreens, etc is pretty much solid white.
Regardless of what type of cotton you use, I recommend boiling the cotton for 10-15 minutes on the stove (no, it won't burn unless you heat all of the water out of the pot

), squeezing the excess water out and then allowing to dry overnight. You can fluff the cotton in the mean time to speed up the drying (just be careful since boiling water is going to be absorbed into the cotton).