I've heard experienced growers say organics are fine up to harvest. It makes sense to me although I think that if I'm staying close enough to the plants to see them start to swell I would back off anyway because there should be enough food left in the soil that along with some molasses you're good.
I believe you want blackstrap molasses and any health food store or organic like Whole Foods carries it. Blackstrap is cooked a step beyond regular molasses so that it's less sweet but more nutrient dense- I'm not sure all the nutrients are bio available so maybe that doesn't matter. Someone with more knowledge might chime in. If it is just for the carbs I imagine any molasses is fine.
Molasses is a pretty common cooking ingreadiant, found in the baking isle of most large super markets. It is also a common farm staple, sometimes used to coax cattle onto a different feed or hay. It should NOT be hard to find.