Buy a 10L pack of Perlite granules. The pack weights about 800 grams.
Perlite is used to improve water retention, drainage and aeration of potting media. This is similar to what Vermiculite is used for with one important difference. Perlite provides better aeration while Vermiculite offers superior water retention. It can be used standalone as sowing media for seeds or in potting mix for general purpose use.
Perlite is good for seed germination both as stand alone potting media or in a potting mix.