Ageratum, I believe. There are both annual and perennial varieties.

This plant can actually be perennial in its homeland Central America, but was normally cultivated as annual in other parts of the world. In 'old days' it was called Ageratum mexicanum, now I can see it being called houstonianum? It has become a noxious weed in many countries and is very toxic for grazing animals .

Ok, thank you for the information. It's definitely behaving like a perennial at my workplace because it comes back year after year. However it seems to be fairly well contained. I think it looks beautiful, but I cannot find it at any of the nurseries.

There is also Eupatorium coelestinum to consider, a native option that should be hardy where you are. This says it can get 3 feet tall but I've never seen that in person. It's usually about 12-18". If your CM is a tree form, that might work for you.

Here is a link that might be useful: Google of images of Eupatorium coelestinum

Many nurseries are stocking fall annuals like pansies and mums now. You may want to wait around till Spring and see if it comes in or just ask people at the nursery whether they typically get Floss Flower (Ageratum) in Spring.

It is also ridiculously easy from seed and you are more likely to find it in that form (at least in the UK unless it is a very short form used a bedding plant).