Hi JAS,
The Tempurpedic 8" Contour (which is the one I’m assuming you mean) had a .8" comfort layer of Tempur material (5.3 lb memory foam) over 2.8" of Tempur material (5.3 lbs) over a 4.4" polyfoam airflow base layer (which is @ 2.2 lb convoluted polyfoam).
This means that you have 3.6" of good quality memory foam over good quality base foam. Of course there are many different memory foams and polyfoam used in a base can have different levels of firmness so having a mattress with the same specs in terms of material quality may not produce the same feel or performance so you would need to make sure that the pressure relief and spinal alignment was good on any mattress you are looking at but this at least would give you the basic construction you are looking for.
Some of the better online memory foam options I’m aware of are listed in post #12 here and I would look for memory foam mattresses that have comparable specs and then talk to them about the “feel” of the mattress.
I would also consider a local purchase because you could actually test the mattress to make sure it was suitable in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences) before buying it which would be less “risky” than an online purchase.
If you let me know the city or zip you live in I’d be happy to let you know of any local options I’m aware of.
Phoenix