Why do you need all of your sites on separate class C IPs? I am just assuming SEO. Unless you have ARIN based justification for this requirement (SEO is not ARIN justification), I would assume most hosts will not do this since it is inefficient use of IP space. You might want to make a request for this in the request forum to see if a specific host can help you out.
There are hosts that are offering IP space like this for SEO services but you will find that they are old hosts that in many cases have an excessive amount of old IP space, many times that have issues in the past with their IPs, hosts with more recently acquired IP space may be able to do this for you but expect to pay a premium for the IP space (this should still be less expensive and certainly less time consuming than dealing with 20 different hosts.)