@Troy ELD With Extreme Lighting Displays in AU sells coro with this extra strength in flutes I believe.
I personally have used 5mm coro from Bunnings in my latest icicles matrix build. 5mm is fairly rigid and with a conduit frame it supports pixels very well.
I made one of my first props out of 3mm coro, but had a conduit frame to support it and it was fine, but as others have advise, 10mm appears to be the "standard" for most vendors. Bunnings sheet were available in 3mm and 5mm if you want to DIY.