As far as I can tell crucial.com shows a total of 2 options. Both 16GB UDIMMs. One tall, one short. I think last time I checked it had none. I didn't see a single option on memory.net. Remember that you need Unbuffered/Unregistered ECC (UDIMMs with ECC) and there aren't many options of those. RDIMMs will not work.

It's also hard to verify the DIMMs will work because the motherboard's QVL doesn't list any of the ones I've found so far. Not all retailers will have these either, so that seems hard enough to find for me.