There are many places where you can obtain IP addresses. It's going to cost you depending.
What region are you in, and do you currently have IP space directly with your provider or do you have an account with ARIN, RIPE, APNIC etc?
Are you looking to actually 'own' the address space or do you simply want to lease/rent it. Either way expect to pay a premium if you can't get more address space from your provider.