What's special that you need to use an expensive probe? The main reason for a 50 ohm probe is because it can reproduce a very high frequency signal, orders of magnitude higher than you'll ever be able to sample at.
These things are usually active and need thier own separate power source. They won't produce many volts so you should be able to wire it straight in with a 50 ohm terminating resistor unless you are wanting maximum resolution.
Evan