I added a milkweed plant to my already crowded garden about five years ago. They've quickly spread and I found my first caterpillar three years ago.
Yesterday, when I was cutting the grass, I had to move a dying milkweed plant out of the way and a monarch caterpillar fell off the leaf. When I looked under the leaves of that plant, I found two more caterpillars. So now I've got all three in caterpillar cage, adding in fresh milkweed leaves, hoping for butterflies.
If you want to help, it's easy to get milkweed to grow in your yard. Here's more info.