There are a few places you can buy IPv4 address. You can buy them from ISPs, data centers, or other companies that have them. You can also find them for sale on the internet, but be sure to do your research before buying from anyone. There are a few things you need to consider before buying an IPv4 address. 

First, you need to make sure that the address is not already in use. You can check this by doing a WHOIS search. If the address is in use, you will not be able to use it. 

Second, you need to make sure that the address is not on a blacklist. Blacklisted addresses are not able to be used on the internet. 

Third, you need to make sure that the address has not been reported as stolen. Stolen IPv4 addresses can be used for malicious purposes and can get you in trouble. 

Fourth, you need to make sure that the address is not being used by a spammer. Spammers can use IPv4 addresses to send out spam emails. If you buy an address that is being used by a spammer, you could get in trouble. 

You should also consider the price of the IPv4 address. They can range from a few dollars to a few thousand dollars. It all depends on who you buy it from and how many you buy. 

Make sure to do your research before buying an IPv4 address. There are a lot of things to consider.