I use either:
* lead flashing molded into briquettes, or laid as sheet between frames or cut into small pieces (very tedious) ; the latter method is ideal where there are small compartments into which the cut pieces can be poured and retained with automobile GRP resin.
*the best , though more expensive material , as far as I'm concerned is No. 6 buckshot which can be purchased from your local gunsmith in tubs of between 5 - 7kg. This stuff pours like water and is best resined in place to prevent it sloshing around in the bilges.
* also try your local tyre supplier for scrap wheel weights though these will have to be melted down and made into briquettes before use.