Im confused by your question.

It is not too late to plant nursery or supermarket mums which are not bred for winter hardiness. look for ones that still have closed buds and they might bloom for month. it doesnt matter when you plant these because their roots dont need a whole growing season to mature in your garden.

Hardy or perennials mums are a different animal. They are bred to withstand winter freezes and return the following spring. Bluestone perennials still has a good variety of perennial mums for sale. If planted now i dont know if they be able to establish an adequate root system to make it through winter.

You do know these would be small starts and not big pots for fall color?

they are not terribly expensive so you could get a few and mulch them very well and X your fingers.

Avant Gardens in Dartmouth MA,if it hasnt closed for the season, carries several unusual mums I never see in a nursery or on line, if you cant find these hardy mums now put a note in your calendar to purchase next summer. They are not usually in the spring catalogues because they are not big enough to ship.

Then buy yourself some pots for $ 3.50 and enjoy the color for A month.

ps advant gardens is open and shipping. They have hardy mums left. Their shipping is very reasonable; it might be worth trying this fall.