Where can IP blocks be hired from on a monthly basis?

I am looking to use a Justified and >95% Class C in the US. I have access to routers at my end for this.

Who else has experience of this?

http://www.arin.net/registration/index.html

IPv4 Allocations to ISPs
Most ISPs receive IP address space from their upstream ISP, not directly from ARIN. Provider-independent (portable) addresses obtained directly from ARIN are the least likely to be globally routable. Therefore, ISPs should consider the following order of priority when requesting IP address space:

Request IP address space from upstream provider
Request IP address space from provider's provider
Request IP address space from ARIN (not guaranteed to be globally routable).
The minimum block of IP address space assigned by ARIN is a /20. All ISPs that do not meet the requirements for a /20, or need allocations smaller than /20, should request address space from their upstream provider.

ARIN has a policy which provides for multi-homed organizations that have efficiently utilized a /21 to be allocated a /20 if they agree to renumber out of the /21 and return the space to its original provider. An ISP is multi-homed if it receives full-time connectivity from more than one ISP and has one or more routing prefixes announced by at least two of its ISPs.