The first result for in google for Tektronix 455 gets
http://w140.com/tekwiki/wiki/455You can find the manual there and a few other links, but there is a wealth of info by googling and on youtube.
So the first couple of things to learn when looking for probes is almost essential to understanding the oscilloscope. It's like knowing what fuel your car takes.
Attenuation (this affects the voltage of signal the scope sees) it's common to get switching 1x 10x probes.
Bandwidth (a frequency performance indicator) your scope is 50MHz so a probe over that within reason is fine.
Compensation (used to adjust the probe to the scope) your scope is 20pF so if a probe adjusts over 20pF then that will work.
Impedance (used to match the probe to the circuit) your scope has 1M ohm inputs
That is how you determine that a 50Mhz+ 1M ohm, switching 1x 10x with at least 20pF compensation probe is going to be what you need.
Something like this should be fine, your oscilloscope has two channels (so two probes if you want to use both at once).
https://www.tequipment.net/Elenco/P-2E/Standard-Passive-Oscilloscope-Probes/Accessories options that come with the scopes are normally found either in the manual or in the sales blurb. Go look up the part numbers if you're interested in getting some secondhand originals. Having a few spares laying around is handy anyway.