laked granite chicken grit (“starter” is the finest grade for small seeds) is available at all agricultural supply stores. It is inexpensive but comes in large and heavy bags. Perhaps you can get some friends to share the lifetime supply you’ll get for about ten bucks, but it can also be used as a potting soil amendment for improving drainage. (I used to be able to substitute parakeet gravel, but lately the product seems to have changed and is no longer simply very coarse sand.)
Filling the pots to the brim reduces the area where air might be trapped along with pathogens. The inert grit is an inhospitable material for fungal growth.