I’m late to posting this but just in case it helps...I’m logging in from an iPad so the desktop version could be completely different.
Step 1 make sure you’re signed in
Step 2 click on the “article” about new 2019 itineraries
Step 3 when the article opens there is a small blue box that says advanced bookings available and you click Check Availability
Step 4 that opens all the available cruises through 2020 and you can filter your party and search there
Normally when I open Disney’s site I can search straight from the main page, when I did that today it would only show through September...the above may only apply to mobile devices but it’s not really intuitive