This is a great question.  While soil is often abundant and inexpensive for most gardeners to use for hilling their potatoes, different types of mulch and straw are also used.  Moss would work well to hill your potatoes.  You could use unsalted peanut shells, especially if they were rinsed/washed in a bucket first and than stepped on/flattened/crushed prior to placing around your plants.  The purpose of rinsing the peanut shells would be to remove any peanut remnants and avoid attracting insect pests or rodents.  Both of these hilling options would be synonymous with using mulch.  I hope this helps, if you have more questions, please feel free to email me at <personal data hidden>

Thank you so much for your answer. I will definitely use moss and washed peanut shells, because buying soil (of any kind) is pricey here in rural Alaska. 

Happy to help!