For black "sand" (grain size less than 1mm) the only source I managed to find was
All pond solutions 5kg bags. At £10 ago it seems pretty expensive (I remember when it was half that price and it wasn't that long ago) but looking round, larger grain sizes seem to be the same price but in bigger quantities. It's free postage and I ended up needing more when I upgrade my tank so that was a bonus. Also probably prevented the postman getting a huernia.
I've not looked into it much but I think that the APS black sand above is just iron silicate same as blasting sand. It's certainly magnetic as I accidentally dropped a magnet on to it.
Under the sand I use a mix of leaf mould, cat litter and miracle gro slow release pellets all held in the fruit bags you can get from super markets. I think they're 50p for 2.