Requirement for a high-speed (10 Mbit/s) PCI-bus synchronous serial port
SeaLevel Systems makes a synchronous board that does 10 Mbit. Have not used their products but they have been in business for at least a few years. Even if they don't have a native .NET interface, keep in mind that you can call 'unsafe' code from .NET environments.

General Standards also makes a board. They are pricey.

Another thing to consider is using a PMC card on a PCI carrier. Note that this is at least a $1500 solution.