Autumn nectar sources are important to Monarchs in migration, but also to all sorts of pollinators that are slowly running out of food options before winter sets in.
One of the best autumn nectar sources is New England Aster. It's one of the Monarch's favorites. I've seen a dozen Monarchs on a single plant in the past, and that's why I added it to my native garden. I had about 6 last year, and then I heavily seeded more over winter and now I have a ton of them. They grow great from seed, which means it'll definitely be one of the main plant species I sell on my plant stand next spring.
I've read that goldenrod supports more pollinators than just about any other type of native plant. Most goldenrods are super aggressive, but Stiff Goldenrod is less aggressive and better looking (in my opinion). The leaves are very smooth. This is a small one that I didn't even know existed, and it's the first to bloom.
Sneezeweed is an attractive flower, even more so considering it blooms in a time of year where most everything else is fading away.
Lastly is Mexican Sunflower. One of the only non-native species I grow because of its importance to local wildlife. It's also a native that has to be planted every season, which means I don't have to worry about it escaping into the wild as other non-natives do. This time of year, it's frequently visited by migrating Ruby-throated Hummingbirds as well as Monarchs. I saw this Painted Lady on it this morning, and I also frequently see Eastern Tiger Swallowtails visiting it.