You can make your own buy creating a 3 sided wooden box, or buy the stryofoam one, and then mount a board the width of your box to the ceiling joists. Then slide your beam over the mounting board and secure to the sides of the mounting board with screws or a brad nailer. https://www.youtube.com/results?search_query=how+to+create+a+faux+ceiling+beam
Here's how we made ours:
Lot’s of tutorials out there. You will need to decide the look you want- finished or rustic? painted or stained? size? number? ability? cost?
These things will affect your design plan. My response isn’t much help, but as you narrow down your plan, take it a step at a time. I and others can help with more specific questions.
There are hometalkers you you can hire for advice and actual assistance.