You typically buy them from your ISP or hosting service. But there are 3rd party services like anyip. Typically with large blocks you will need to provide justification for them because IPv4 addresses are somewhat scarce especially US ones.
It will be tough to get that many and if you can it will be very expensive. The only one who might possibly be able to rent that many might be anyip as mentioned above. I do not think they rent that many to one person but I might be wrong.
I can't imagine that you need that many IP addresses in one location. If you have multiple locations it will be very hard, if not impossible, to get a consecutive block of them.