Bettina,

Thank you for using MSU Extension Ask Extension with your cattail question. First know that fresh cattails are available through most florist suppliers. 

As for cutting your own, It is illegal in Michigan to remove aquatic vegetation along shorelines without an EGLE permit.   However, on private property in a ditch for nuisance plant control cutting and mowing is likely ok.  I advise contacting your local DNR office to verify.  https://www.michigan.gov/documents/dnr/DNR_customer_service_guide_407568_7.pdf 

Putting those items in a  commercial product (floral arrangement) meant for sale is another matter entirely. Again, check with the DNR. Additionally, I caution that you only use Michigan native plant materials in your products because using a non-native variety may accidently spread this very invasive Plant. Learn to ID the difference here: https://www.habitatmatters.org/narrow-leaf-cattail.html and http://www.annarbor.com/home-garden/cat-tails---an-unsung-native-invasive/. Native choices while cheap and easy to get often come with fungi, diseases, unwanted insects, and spiders. These are not desirable in a purchased flower arrangement. Even it it is from a farm stand.

Do not use any animal parts, or bird feathers for your art or arrangements.  This falls under US Fish and Wildlife laws https://www.fws.gov/international/pdf/factsheet-can-i-sell-it.pdf  It is better to purchase legal or reproduction materials from a craft supplier for these elements. Get a good ID app or book to help you avoid any endangered plant species, as well.  it is illegal to collect threatened and endangered plant species without a special collection permit. 

This website has more information and contacts about collecting natural resources for commercial purposes. https://www.michigan.gov/dnr/0,4570,7-350-79134_82777-230545--,00.html and https://www.michigan.gov/dnr/0,4570,7-350-79136_79262_80436_85610---,00.html  both are resources for more details. 

Bottom line: On private property that is not along a shoreline cutting any species cattails is generally ok without a permit. Using in a floral arrangement that is for sale may need a permit.  I do not know for certain.  Check with the DNR before proceeding.  

Note: Cattails are available commercially through many florist suppliers. I recommend this as your best option.