You will not find any. Developing secure microprocessors is very expensive, and I doubt anyone would expend a truckload of money (literally) and open source it afterwards.
Even if you don't develop your own microprocessor, you will have to license one, and that will come with a NDA, because the company licensing you one will not want you to disclose the innards of their processor. You could risk a hefty fee if you disclosed it.
I can't place much trust in a black box that was mailed to me without anti-interdiction services and no way to verify the integrity of the hardware.
You are not putting your trust on a random blackbox that arrived by mail. If that blackbox comes with a FIPS certification, you are putting your trust on the certification.
It's like your bank: you have your money there, and you are trusting a black box full of smaller black boxes that will take your money and say they will take care of it. But if the bank has a certification from the government, you can trust the certification, and the bank earned it by proving to someone that they can indeed keep your money safe.
Likewise, the certification guarantees that, as far as FIPS can tell, that blackbox withstands all sorts of attacks without leaking the private key, and that's enough for me.