I suggest doing a search for "California native plants for sale". Pick a nursery close to you, or make it a road-trip destination. They could advise you on what would work well.
Milkweeds are for Monarchs, look for a variety that does well in your area. Don't forget to plant nectar plants also. Are you trying for a Monarch Waystation?
Where will the garden be located? Is it at your home or in a public area? What sort of soil does it have, also how much sun? Will there be some sort of watering system? Are you interested in starting some plants from seed? Is there a time limit on getting this going?
Sounds like a great project
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