Hi Eyeone,
Welcome to the Mattress Forum! 
The uppermost layer of any mattress will soften the most over time and undergo the most mechanical stresses, but it is odd for latex to “sag” in such a short period of time, synthetic, blended or natural. I’m not sure what you mean by 10% Dunlop – perhaps a blend of natural and synthetic latex?
One thing I always advise to double check when I hear about any mattress sagging in the center is to make sure that your bed frame has a proper center support, and that whatever foundation you’re using for your mattress is appropriate and not sagging as well.
There are quite a few sources for Talalay toppers listed in post #4 here, and you can also look at the component post listing here. MattressToppers.com is one of the vendors on this list (their products aren’t return or seconds), and they are a member of the site. And SleepEZ offers Talalay from both Latex International and Radium (19-20 and 30-32 ILD, both blended and natural), so you’d have to check with them regarding which manufacturer was supplying the layer in which you had an interest.
I think that covered your questions from both of your posts, but if you have others, please let me know.
Phoenix