Looks like Intel (prev Terasic) is still selling them but they're now around $300, they used to be under $150 if memory serves.
If I were jumping into this today, I'd prefer the much more powerful KV260[1] from the blog post over the DE10-Nano, that board was limited!
[0]https://www.intel.com/content/www/us/en/developer/topic-tech...
[1]https://www.amd.com/en/products/system-on-modules/kria/k26/k...
Sidenote: somewhat sad to see the consolidation into Intel and AMD, but FPGAs have always been second class citizens in the silicon space :(