The probe tips are just (very special and tightly controlled) 25k resistors connected to short bits of coax. You can test the probe by connecting a 25k or similar resistor from a test signal source to the center pin of the connector. There's a little hole for a pin for a ground connection on the probe amp head.
Edit: With the resistor connected to the + input you should see the signal, connected to the - input you should see the same signal inverted.
Here are the probe tips and accessories you want to be looking for on Ebay:
E2675A Differential Browser Kit for InfiniiMax Probes
E2669A InfiniiMax Differential Connectivity Kit
E2668A InfiniiMax Single-ended Connectivity Kit
N5381A InfiniiMax II 12 GHz differential solder-in probe head
N5382A InfiniiMax II 12 GHz differential browser
N5425A 12GHz InfiniiMax differential ZIF probe head
E2677A InfiniiMax 12 GHz differential probe head
N5451A InfiniiMax differential long wire ZIF Tip
E2679A InfiniiMax single-ended solder-in probe head and accessories
E2676A Single-ended browser probe head and accessories
E2678A InfiniiMax single-ended/differential probe head and accessories
N2880A InfiniiMax In-line attenuator kit
Probes themselves:
1130A InfiniiMax 1.5 GHz Probe
1131A InfiniiMax 3.5 GHz Probe
1132A InfiniiMax 5 GHz Probe
1134A InfiniiMax 7 GHz Probe
Here are the search terms you can dump into ebay to find these:
Probes:
(1130A, 1131A, 1132A, 1134A)
Differential
(E2675A, E2669A, N5381A, N5382A, N5425A, E2677A, N5451A, E2678A)
Single Ended
(E2668A, E2679A, E2676A, E2678A)