You can make your own organic, peat-free seed and potting composts using leaf mould, garden compost and loam. You can make it up as you need it, in large or small quantities. Leaf mould needs to stand for two years. Loam is basically just good garden soil. If you are bothered by weed seedlings you can pasteurise the loam at 80 degrees C for about half an hour. I just pull the weeds up.
The recipes are by volume: shovel-fulls, scoops, or whatever.