There are two places in the Bay Area, peninsula specifically, that I always get it from. Dittmers in Mountain View and Village Market in San Francisco. Dittmers charges $5 per ounce ($80/lbs!!!) or $24/lbs for more than a lbs. I faxed Dittmer to fix this pricing error so we will see if he updates the pricing. Village Market has it for around $5/lbs which is more like the price you should be paying for something that is 93.75% table salt and 6.25% sodium nitrite.
As a side note, the recommended amount of 25lbs per ounce is only for a dry cure. If you use a wet brine you must follow the recipe. I have used a recipe which called for 1.5 ounces for a 4-5 lbs brisket using a wet brine.