Ascorbic Acid

in Peter Reinhart's book "Whole Grain Breads" he recommends using ascorbic acid and i've seen it in bakery whole wheat loaves in the ingredient list as a "dough conditioner." I'm sure as you all know ascorbic acid is basically vitamin c, but i'm just curious as to what you would use as your source of ascorbic acid.

Does anybody know, or have some suggestions or ideas? I'm really clueless on this one, hope to hear from you guys very soon because I'm going to be baking another loaf of bread soon and would like to try this. I appreciate all your help in advance.

-Dan