Version 2.1 includes additional information for the transfer of real-time kinematic data. SC-104 Standards document may be purchased from the RTCM or from Navtech Seminars. Addresses for both are listed here . Sometimes the only way to get what you want when you want it. Not necessarily all that expensive, either depending on what you are trying to accomplish.
Some organizations maintain base stations for post-processing and are willing to share the data.
Check around state Department of Transportation offices and civil engineering and surveying departments at universities. The U.S. Coast Guard has tested a system of high frequency DGPS radiobeacons positioned around the coast and along certain waterways. Starlink Incorporated maintains a list of sites and system status for the Coast Guard system and for systems for other countries. This information will be sent to two communications satellite earth stations and relayed to the aircraft via a satellite signal that is similar to a GPS signal with unique codes. This signal may also be suitable for ranging providing improved navigation availability.