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.
That is a very good question and not only for cloud providers but also for businesses with /8 ranges and even smaller ones which currently are valued at $1+ million
I'm not aware if there is any business that really registers them as assets or any tax authority requiring this but I'd also be very interested to hear more on this
I can't speak to how all transactions are conducted, but there are auctions on one marketplace, IPv4 Global auctions, with cost per IP in the $50-60 range https://auctions.ipv4.global/. I also really enjoyed Flyio's blog post about acquiring addresses https://fly.io/blog/32-bit-real-estate/
You can't buy smaller than a single /24 through the officially approved arin, ripe and apnic transfer processes, since that is the minimum size of individual netblock you can announce into global routing tables.
An asn is not something you buy from a third party unless acquiring a whole corporation that already has one, since there is no real shortage of them, you'd get your asn from your regional registry.
