Yes 4K set-top boxes are available online.
I have bought a 4K set top box on ebay but you can usually find them on most other websites such as Amazon and other websites that sell electronics.
Some stores such as TV stores or other electronic stores usually ave those 4 K set top boxes as well.
Or you can buy a 4K TV which is what I plan to do in a few years when I save enough money.
In a few years my current TV will likely need replaced anyway so I'm gonna buy a larger 4K TV and not have to use the Set Top 4K box.
I will likely sell the 4 K TV Set Top Box in a few years on Ebay.
You might also find some used 4 K set top boxes on ebay as well cheaper than new ones that work just as great.