Ten dollars a lb is a good price. These are the cheapest I have found so far. Here are two links:
http://www.msm-msm.com/store/agora.c...scorbic%20Acid

http://www.chemistrystore.com/Ascorbic_Acid.htm

I don't dilute the ascorbic acid with water. I just go around the pool and drop it down the sides with the pump on circulate. I have a 16,500 gal pool and only use 1 to 2 lbs for a treatment. Leave the pump on circulate for about an hour. Make sure you put in a sequestering agent. After an hour, or when all of the stain is lifted, turn the pump to filter and leave it on 24/7. After one day start to raise the ph. After 48 hours start to raise the chlorine slowly - if you have a sand or DE filter put a trichlor puck in the skimmer. DO NOT do this if you have a cartridge filter. Don't shock for at least 2 weeks.