A quick search for "hydroponics stores in " your area, results in a few shops that sell it in small blocks.
I don't know if I'm missing something but I checked all their sites out and they all sell 1.5" cubes in reasonable quantities but of the stores I checked not one of them sells the mini-cubes (which are what I thought this thread is about?) in quantities under 2 cu ft/$45 which are too many for me!
I guess I misunderstood that you were looking specifically for small cubes. Your question was about where to get small amounts of rockwool. The stuff cuts like butter. I know of one you-tuber who buys it in 4" blocks and cuts her own small cubes as needed during her repotting sessions.