I'd like a wide opening for my pole barn... no roof yet. It is 19.5 feet wide from inside of one pole to the other. They're telling me I need to put in additional posts for framing which would narrow the opening. I really want a barn door that slides from side to side...
I don't want to narrow the opening.
Any ideas?

Get a used 18 ft. double garage door or 2 X 10 ft. single doors, bolt together, mount barn door rollers to it and a track to the barn. Extend the sides if needed. A cheap hanger would be garage door rollers on brackets and a garage door track. Check Craigslist.

Sounds good but what about supporting the doors above? I'd need a header right? Would I need any support on the sides... framing in?

That would depend on the overall construction. Yes, a header for sure. Look on line at "framing a sliding pole barn door".
# How To Frame A Pole Barn Sliding Door #