There are different things of what people call a marker generator. If you want what I think you want, then for that type of marke ryou need a (triggerable) 1 kHz pulse generator. Almost any kind of pulse generator would do, depending on what precision and accuracy you want.
Then you have two issues:
1) How to synchronize the marker generator's phase with the signal you want to measure
This really depends on what you want do. E.g. do you need synchronisation at all? If yes, and if your oscilloscope has a trigger-out you could use that to start a 1 kHz pulse train from your marker generator. This requires your marker generator can be triggered. Add a bit of adjustable time delay and you can position the marker signal's phase relative to the signal you measure.
If you instead want to have a marker in a sweep then your marker generator would have to be triggered on the sweep control signal, often a sawtooth signal, instead of the oscilloscope's trigger-out.
The most simple way I know to add a kind of trigger circuit to a generator is to kind of fake it. You use a contentiously running oscillator at a frequency much larger than the marker frequency you want. You run that high frequency through a resettable divider. The trigger signal goes to the reset input of the divider. You get some phase jitter and glitches at the output, but it might be good enough for your purpose. Oh, and you need to have some pulse forming at the output of the divider.
2) How to couple the marker generator output into the oscilloscope
If your oscilloscope has a Z input you use the marker generator output to blank or intensify the signal at the marker position. Alternatively, if you have a two-channel oscilloscope you could just display the marker signal on channel two. If that is not possible you could mix the input signal and the marker signal. If that is not possible you could open your oscilloscope and add a Z-input or blanking input.