What are appropriate probes for use with a HP 54501A Osciliscope
Any "classic" scope probe will probably work.  Here's one for pretty cheap http://www.adafruit.com/products/1528?gclid=CMXu85Hkj8ICFeLm7AodEEIA3Q

According to the manual the probe it came with was this guy http://www.keysight.com/en/pd-1000000970%3Aepsg%3Apro-pn-10432A/101-1m-miniature-passive-probe?cc=US&amp;lc=eng

Depending on what you want to measure you can also make your own  with just some 50 ohm bnc cable, or by making an actual probe like this http://cromwell-intl.com/radio/probes.html

But as cheap as you can get these I'd just buy one.