baking isle in grocery store if you can't find blackstrap ,just get any unsulphured i do 1 tbl spoon per gallon.
While I would never use it, you can get it at the grocery store.
Find un-sulphured stuff without any salt or any additives in it because that will mess up your grow almost instantly.
Take the time to go to a health food store to get the good stuff. Yes, it will cost more.
Truthfully, it's better to use something from an MJ nute company since those products are made to help plants grow. I'm just not convinced molasses is designed to help weed.
Ain't in my grow room.
good luck dude