You can't necessarily buy a certain IP, but there's a lot of proxy providing services that offer packages that have main gateways where the IP's remain static and won't change.
You can shop around from different sites and check out different packages depending on what you want specifically, but you won't be able to buy just one location/IP/Gateway.
If you're just looking for a static IP to keep to play on a few accounts and no others, I'd recommend to just change your IP.
The easiest way is to just change a few numbers in your MAC Address, contrary to everyone saying to unplug your router for a few minutes.
There's guides all over on Youtube on how to do it, but if you need help feel free to PM me.