You will not be able to access any genuine credit rescoring service directly. If you hear of one advertising to the general public, you are very likely about to be scammed. Mortgage lenders work with rapid rescoring services on an individual basis. For example, if you have a collection on your credit report that was the result of identity theft, your mortgage broker can consult with a credit rescoring agency. They have the clout to get in touch with the credit bureau reporting the error and in many cases get it removed.
It is important to remember no credit rescoring service can be guaranteed. The process can take anywhere from three days to two weeks, and will go much faster if you have written evidence to back up your error claims. However, rapid rescoring can work much faster and easier than an individual consumer writing letters to credit reporting agencies and waiting for well over a month for the results.