Shipping dirt is not very economical and there is always plenty near by.The cheapest way would be to buy a large bag of perlite or vermiculite along with a large bag of peatmoss.Mix this 50/50 and you have a generic growing medium that will give you the best control features do to its makeup.I fill my trays all the way up and get my supplies at a local greenhouse.lowes has these items but are starting to phase them out at this time of year.You can always use plain old potting soil when in a pinch it all depends on how much work you wish to do in your growings on.So how has your flats been turning out,nice growth with high yields?