I purchased the sulfur powder from Amazon.com: Sublime Sulfur by Humco
Edited to add that you generally want your sulfur mix to be less than 10%. Too much sulfur can cause scalp irritation and be very drying for your hair. This mix is 2% sulfur.
I purchased the bottle from [here] for $0.89 and unfortunately, the company that sells the bottle does not carry applicator nozzles, so I took the nozzle off an applicator bottle I bought from Sally's for $1.00, shown [here]
Note: Be sure to stop using all sulfur products at least a week prior to a relaxer application and do not resume until at least a week after you relax, to prevent burning/irritation. Also, do a spot check to see if you're allergic to sulfur prior to applying the mix to your entire scalp.