In my quest for a cheap source of borates that did not involve gallons of muriatic acid, I came across http://www.galladechem.com/.
They have 50 lbs Granular Tech Grade Boric Acid (p/n B10600), but you have to contact them to get a price.
The responded to my query with $44.50 for the bag ($0.89 /lb).
They also claim to have "free shipping to most of California", but they told me their trucks don't go to Sacramento.
They quoted me $34 for shipping, so this is still a pretty good deal, but if you live within their free shipping area, or close enough to one of their 4 facilities to pick it up, this would be a great deal.