Now you just need a source. Walk around your neighborhood and community gardens looking for the plants you've identified. Ask if you can take a cutting or get a division in the fall or spring. Pachysandra spreads well from cuttings. Hostas need to be divided every two years in the spring.
Forsythia grows by just sticking a cutting in the ground and watering it.
Also, look out for people preparing to do construction work on their property. They're often anxious to get rid of them and will give you a pile for even less than the price on the pot. Finally, don't forget to collect seeds in the early fall. Marigolds grow easily from seeds and are easy to harvest. Many homeowners would probably be glad to let you have a few of the dried up heads full of seeds.