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. There's even a free weekly email newsletter. All rights reserved unless specifically noted. All advice should be weighed against your own abilities and circumstances and applied accordingly. It is up to the reader to determine if advice is safe and suitable for their own situation.