It's not out of stock, you just have to buy it at the market price, which may be more than you're willing to pay.

I just bought raspberry pi 4 on amazon through canakit. It came with a bunch of other stuff that was useful like sd card, case, cable, hdmi mini to hdmi converter, etc, so it really wasn't that bad.

Here's the equivalent for rpi zero for $50

https://smile.amazon.com/CanaKit-Raspberry-Wireless-Complete...