Kimberley Morrison​ Are you working with Nintex Forms for on-prem or Office 365? I've recently built a NCAA march madness bracket solution which leveraged cascading jQuery drop-downs and believe I can help!

If you don't mind wading through a little back-and-forth dialogue, the following thread has some good insight into the process as well as some sample code:

NCAA bracket in Nintex Form

I'm planning on sharing a similar solution soon on the Xchange Marketplace, so that may be a good starting point as well if you're requirements necessitate a javascript based approach.

Hello Kimberley Morrison​,

In case you're still in pursuit of this solution, I've recently blogged on the topic and have a sample form available that should help!

How to Create Cascading Choice Controls – Nintex Forms | Summit 7 Systems