I was doing a genocide run and I got stuck because I don’t know where to find the monsters I need to kill to progress
Outside of scripted encounters, monsters in Undertale attack randomly while walking around in any areas which can have encounters. For Snowdin this includes most places before Snowdin Town baring a few special locations.
If you've already done genocide up to Snowdin then when you hit a save point it should inform you how many random monsters are left in the area with a red number. Once you are done, the save points will say "Determination" instead and you'll get the no-monster encounters, so you can do this to verify that you're still on track.
Keep in mind that as you kill more of them the encounter rate will get lower and lower, so expect it to take a while.