The street stalls of every major town in Viet Nam are awash with modern reproductions of the classic NVA green "pith helmet".
REAL ones are VERY hard to find for a number of reasons.
1. They were officially withdrawn from service almost a decade ago. They were replaced by peaked caps for "general" wear and proper steel helmets for "serious" work.
2. Most of the "originals" were worn by farmers, cyclo drivers, or just about anyone, until they fell to pieces.
3. I got a "pre-loved" original there, complete with a scruffy original badge, about eight years back. It now resides in a military museum in Brisbane.
4. The "trade" helmets of my acquaintance are NOT made from a soft "pith", either natural or synthetic, but are a simple hard shell of an unknown plastic material. The "liners" and chin-straps of these are "cheesy" to say the least.