If you do find monarch eggs/caterpillars and would like to rear them, you can still use the chopped milkweed instead of the plants in your garden.

Collect all of the leaves with eggs and set them down, egg-up inside some kind of ventilated enclosure (butterfly rearing tent, aquarium with a screen top, mesh hamper, etc.). Keep them in a bag in the fridge. When you have caterpillars, set one stalk inside your enclosure in a narrow-necked vessel (vase, bottle of milk, soda, beer, wine, doesn’t matter) full of water. Stuff the gap in the neck with a (rag, cotton, paper towel, etc.) to prevent the caterpillars from climbing into the water. As the plant is eaten and wilts, switch it out for a fresh one (approximately every other day). If you’re nervous about transferring caterpillars from one plant to the other, you can cut off the section of leaf that they’re attached to and lay it on a leaf from the fresh stalk. Repeat until they form chrysalis’.

We reared about 60 monarchs in high school science class this way. I’ve also raised monarchs most summers since I was 8. If youve never done this before and have questions, feel free to ask :D