Can you elaborate on the "rubber"? I think that's the one most Vietnamese are after, my wife is no exception 
Hello,
Newbie here, greetings to all.
I believe this is what some of us called Chewy Sugar Apple. It's the one that I've been trying to plant for a few months now ... from seeds. There seems to be a nursery somewhere in Riverside, Socal, that had it on eBay but they seem to have suffered from a "harsh" SoCal winter and have completely disappeard from the eBay scene. I've managed to get some seedlings recently, 3-4 weeks ago, out of seeds from chewy SA from Cambodia. We'll see how that goes.
My plan is to graft some onto Cherimoya rootstocks in hope to get the SA to be a bit more hardy than it originally is.
I've also bought a couple of Kampong Mauve of which one is supposed to be at fruiting age. Next year will tell.