For a /28 you'd have to go to someone that has at least a /24. Probably Colo a bare metal box. If you want to buy a /24 you could go to arin, assuming you're in north America but even that is quite unlikely with the amount of up space left. You'd probably have to buy third party from a broker. Last I checked that was in the 4$ per IP but I haven't been following for a few years.

At this point, most of the free up space is picked up by the big boys and you gotta do business with them.

Airn has a few senarios where you can pick up ipv4 addresses but it's pretty though at this point and you usually would need to be an established business with an existing ipv4 and a dual stack ipv6 setup. The ipv4 they are giving out is basically preferred to providers that are dual stacking and need additional ipv4 space to roll out ipv6.

Honestly, if you were to start today, there's basically no chance unless you gett off your wallet and pay a 3rd party at auction. And those ups are usually going to be trash. You'll spend a year getting them off blacklists and able to function.

So theoretically, you could get some but in practice, you're best bet is to rent them from Someone that already has them and put a router or server in their Colo.