The short answer is, whoever you pay to do it. When you chip any animal, there are two associated costs... 1) the cost of the chip ($25+), and 2) the cost of registering the chip and animal combination with one of the databases that animal shelters and veterinarians have access to.
Even with a centralized system, things aren't all peachy though. Even though a new chip hasn't been released in years, there are multiple standards, not all of which are compatible with the same readers, so even if you chip your animal, there is a chance it won't be seen by the reader being used to scan.