Welcome to car-seat.org.
I'm sorry that you lost part of your car seat! When that happens it is very frustrating! Unfortunately, the only safe kind of replacement part for car seats is a part that you buy from the manufacturer (or a vendor that sells official parts). Have you tried calling Diono and talking to a real person? It is possible that something got lost in translation when you wrote to them, or that you got some sort of automated response that wasn't appropriate for your inquiry. I am not seeing the part you're describing for sale on their website at all, so them saying that it is out of stock doesn't make sense to me.
Whatever you do, please do not try to make one yourself! It may seem like a good idea, but it is a risky business. If the one you make somehow falls short of the official version, then your seat could fail in a crash, putting your child in danger.
ETA: I'm assuming that the part you are referring to is like the shiny metal thing in the middle of this picture, that has the three straps connected to it. If so, it is called a splitter plate. When you call Diono, tell them you lost your splitter plate.
https://www.google.com/search?hl=en...a-glossary-of-common-carseat-terms%2F;540;720