The Rock wool used for insulation hardly absorbs any water and hence probably is not suitable for hydroponics. Also, the one purposed for hydroponics is PH neutralized and properly treated for contamination. However, there is a variety of choice of growing media in hydroponics.
There is no stipulation of using any specific media in hydroponics. Most important thing is to make sure that whatever you use should be inert (should not react with the nutrients). I have been successfully using coco peat since years for all kind of plants. It works great for germination as well as for mature plants.
One option readily available probably in most part of the world is sand. It can be safely used after properly washing. Dutch bucket method is most suitable for it. If you are planning to do NFT, you can simply use regular sponge to support the stem. People have experimented with pool noodles too for NFT.
To conclude, I would recommend to not go for costly options as they do not do any better. Use whatever is available easily like sand, cinder, coco peat, husk, vermiculite etc.