If you need small amounts of sterilized potting soil, you can sterilize it in your oven at home. But be aware that potting soil can create a pretty strong smell (and not a good one!) so be prepared for the smell and keep the windows open while sterilizing and afterwards to air out your kitchen.
In a deep baking pan, spread your soil evenly in the pan, to a maximum depth of four inches deep. Then cover tightly with aluminum foil. Insert a candy or meat thermometer through the foil top and into the soil in the center of the baking pan. Heat the oven to 180-190F, heating the soil to 180F. Do not allow the soil temperature to rise above 200F, as this can produce plant toxins within the soil. Once the soil reaches 180F, maintain this temperature for 30 minutes. Remove from oven, allow to cool, and keep the foil on top to maintain a sterile potting soil environment until you are ready to use it.
If you end up heating your soil higher than 200F, or you have reason to suspect it might have not been sterilized properly, you can check to see if your potting soil is toxic.