You can purchase your garden plants at a nursery and I suppose for the annuals you must, but if you prefer your garden to spring up from the ground each year without spending your money you can engage in the lovely habit of sharing your perennials at no cost.
I had the most pleasant surprise one year after seeing window boxes decorated with Pansies and Pussy Willows. I had been offered cuttings from a neighbors Pussy Willow bush and tried to reproduce the look. To my surprise, one of the stems grew ROOTS! I immediately placed it back in the pot filled with dirt and let it winter over in a protected spot in the yard until last Spring when I planted it in my garden. Today I have my own small Pussy Willow and in a few years will no longer have to take clippings from my neighbor to make these lovely arrangements.