Where to get Differential Corrections
In the early days of GPS, reference stations were established by private companies who had big projects demanding high accuracy - groups like surveyors or oil drilling operations. And that is still a very common approach. You buy a reference receiver and set up a communication link with your roving receivers.
But now there are enough public agencies transmitting corrections that you might be able to get them for free!
The United States Coast Guard and other international agencies are establishing reference stations all over the place, especially around popular harbors and waterways.
These stations often transmit on the radio beacons that are already in place for radio direction finding (usually in the 300kHz range).
Anyone in the area can receive these corrections and radically improve the accuracy of their GPS measurements. Most ships already have radios capable of tuning the direction finding beacons, so adding DGPS will be quite easy.
Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers.