They are assigned to you by the company who provides your dedicated line. So yes if you rented a T1/T3 etc then you'd be able to get a block of IP's. You can probably lease a Class C or half a Class C license or more if you needed them.

Becoming a domain registrar is pretty costly. I use bulkregister.com myself - you can buy domains for $10 and also resell them. You have control of all details such as technical/administrative contacts and nameservers. You can register new nameservers too with the click of a button. In effect you are just using Bulkregisters registrar account instead of buying your own.