40 days out

There are lots of white flowers similar to mums, but it's tough to answer that without knowing where you are, and whether you're growing them yourself or getting them from somewhere - ie, a farm, direct from a grower, the grocery store, a florist etc.

if from a grower, a farm or a florist, they should be able to suggest other options. If you're growing them yourself or buying them from a grocery store, maybe see what they have now that are just starting to appear in the shops, as they will likely still be available in 40 days.

Your last option is just to google your area and what flowers are in season in late october/at halloween, and pick some you like from there.

Are you leaning more towards greenery, or trying to find a while floral replacement??

You can get Raunculus year-round and that could be a good substitute!

Gotcha!! I'd take a look at greenery and baby's breath then that you can buy in bulk and use in multiple ways!