I was able to connect with the folks at Livingstone Landscaping on this question. They've echoed that one wouldn't want to use ordinary topsoil indoors for starting since it can carry fungus bacteria. In the greenhouse industry they use pro-mix or starter soil. Livingstone does bring this in in quantity and has some coming in in March that they sell. If needing sooner they've suggested that they know of Peavey Mart and Green Spot carrying the same. If you think of it, let us know what you end up finding in case there are other folks watching this thread now or in the future that have the same question!