just got a bag last week, 50#'s for about $8 at the feed mill. I think its an additive in some mixed feeds, so if the mill you use mixes it prob has some. It is made by arm and hammer and was called something slightly different (sodium bicarbanette?) but the ingredients are the same as baking soda.
We buy 12 lb. bags of Baking soda at Costco for $4.50. i like the fact that they are waterproof packages and manageable sizes.
Of course, I'll be in trouble when some druggie figures out how to make some drug using baking soda...you should see the looks I get at Costco when I buy 2-3 bags at once!
All our feed stores and TSC carry it here in 50 lb bags for around $10. As someone already mentioned- you might have to ask for Sodium bicarbonate, not baking soda. TSC has it on the shelves where the salt blocks and mineral bags are.