Hi RiclatRichard,
The good news is that most of the Tempurpedic mattresses (although not all) use good quality materials and may be very suitable for “fine tuning” with a topper to provide the extra pressure relief you need. It would be helpful to know which Tempurpedic model you have as well as your body type and sleeping positions (besides just side sleeping if you sleep in multiple positions).
Post #8 here will give you some guidelines which would be helpful in making the best possible topper choice in terms of thickness and material.
I would also tend to stick with the type of material you are most familiar and comfortable with (which is probably memory foam) so that you have less variables to deal with unless you also want to test other materials and combinations in local stores to see how they feel and perform and whether you prefer them over memory foam. A topper will affect how the layers underneath (in the top layers of your mattress) will feel and perform as well so this needs to be taken into account in your choice of topper.
Some of the better online options I’m aware of for toppers are listed in post #4 here although there may also be some local options available to you as well.
Hope this helps
Phoenix