Depending on where you live (I am sure there are non-US equivalents), the local county 4-H and/or state Department of Natural Resources might have pamphlets and even packets of seeds for local pollinators. I added several to my garden and got a lot of neat butterfly species to visit this year.

One thing I learned from my local 4-H is that monarch butterflies only eat milkweed--the linked Wikipedia article clarifies that they only eat from the milkweed genus--but that was a surprising fact. The species within that genus are also not equivalent in terms of function for the monarch's lifecycle, as the linked article also mentions, so it's important to plant an appropriate species with respect to your location.

In addition to milkweed, there many plants that butterflies (and other pollinators) enjoy--I've had great success with zinnias, coneflowers, yarrow, and a few others, too. So it is worth taking a look at the resources in your area and planting native pollinators when you can.