Retail landscape nurseries are going to be your go to for sourcing plant material. You may find that nurseries in the city will charge more than nurseries in the country so it may be good to shop around if you can. Landscape supply stores will sell topsoil, mulches, various types of aggregates, pavers, and in some cases plant material. You may be able to purchase some of your plant material at big box retailers if they have the availability but it is advised to purchase your hardscape material from a proper dealer.