Your motherboard has 4 ram slots and can support 32gb. That would be a 4 x 8gb kit. It is not clear to me what you can not find.
Look for a single 16gb kit made up of 2 matched sticks of 8gb each.
Do not count on being able to use your 8gb stick. Try it, if it works, you get 24gb. If it does not, well, that is why ram is supported in kits only, they must be exactly matched.