Where have you been able to find them? PCPartPicker hasn't exactly shown great availability on the 3900X so far [0], Amazon is full of scalping [1] and Ebay... Well, $700+ is what I'm seeing there. This is the CPU that'll probably be powering my next build, so if you've got a source close to MSRP I'd love to hear it!
0: https://pcpartpicker.com/product/tLCD4D/amd-ryzen-9-3900x-36...
1: https://smile.amazon.com/AMD-Ryzen-3900X-24-Thread-Processor...