I live near a Mattress Store that seems to have a side company manufacturing PranaSleep latex mattresses.  I found one of their mattresses (actually most of them, but the differences were just thicker cores) to be really comfortable, so I've been trying to figure out how to recreate the experience without spending something obscene. 

The mattress I liked so much was a 6" Core (medium firm?) with 2" of  "exclusive super soft 10ILD talalay latex"

I can't find 10ILD latex anywhere.  Has anyone ever heard of it?  I have been trying to imagine how a 20ILD or one of the 14ILD's from sleepincomfortusa would compare as a topper. 

I was thinking of building up a foambymail 33-38 ILD core and 1" 32ILD topper,

then maybe foambymail's 1" 20ILD toppper (though I saw discussion here that there's no difference betweeen their 20ILD and their 32ILD,

and/or maybe a 1" 14ILD or foamorder's convoluted latex which is 2" thick (1" solid) and they say 25ILD Talalay.

I would think 14 ILD would feel very similar.  Some people have mentioned sleeplikeabear.com as a source also.

Yes, I have heard of super low ILD latex; I can't remember if it was ILD 10 or 12. (By the way, many foam experts are now using the term IFD instead of ILD, according to what I found in my research on the net. An expert should know it as either name, however; but an amateur may not, so you may try both names if they go "Huh??")

Anyway, I wish I could remember where exactly I found it - this was a few years ago - but as I recall, I called Latex International and they referred me to someone and he said he thought he could get it for me. However, he never did call me back. So I realize this isn't much help, except to say, yes, I think it does exist, somewhere.

I would imagine that a 12 or 14 IFD wouldn't feel that different.

I'd try calling some of the foam sources you find with a google search and ask them if they know where you can get some.  Don't put more than a couple inches of soft latex - or any other soft foam - or your back may not get proper support.

I think 1", 1 and 1/2" or 2" of soft latex would be enough to soften your mattress without bending your back out of shape.

One thing to keep in mind is that if you're buying 2" it is better to buy 2 x 1" layers instead of 1 x 2" layer. It just gives you more options by being able to try just one or to try both together.