If you’re going to invest time and money in growing from seed, then I think it’s worth using a fresh, good-quality medium meant for seed-starting. When I had a nursery, I’d use Pro-Mix for everything, from seed-starting to seedlings to container plantings, and had great results. I can’t easily get it these days, though, so I’ve tried a variety of other commercial mixes, and I’ve been very happy with the Seed-Starting Mix from Gardener’s Supply so far. It absorbs water readily and is fine enough to make sowing and later seedling-separating easy. Jiffy Natural & Organic Seed-Starting Mix is also very good, and you may be able to find it locally as well as online.
You will probably have reasonable results with whatever seed-starting medium you can buy locally. Start with one small bag and see how it goes. Even the same brand can vary from bag to bag and year to year, so don’t buy more than a few at one time. You may want to switch kinds if you get a bag that seems too chunky or possibly contaminated with fungi or other pathogens (if none of your seeds do well, for example).
I don’t advise using bagged topsoil or garden soil, which can be too wet and heavy for successful indoor sowing. They can also contain weed seeds, pests, and pathogens. “Potting mix” or “potting soil” can be ok, if you can’t find a product meant specifically for starting seeds, but it is coarser than seed-starting mix and may be less than ideal for small seeds.
Pretty much all of the seed-starting mediums you can buy are based on peat moss. If you are concerned about that environmental issue, you may want to try an alternative, such as coir fiber. I purchased a few blocks of Eco-co Coir® Potting Mix from Gardener’s Supply but haven’t tried it yet, so I can’t yet give my opinion on how it compares to traditional mixes.
