For the sour cream, I put 1 cup cashews with 1 TB vinegar (preferably something like sherry vinegar, ACV works too), maybe 1/4 tsp of salt (to taste), and enough water to get to the desired consistency ("as needed"). Blend in a high-powered blender like a Vitamix until smooth.
Can also inoculate with a yogurt culture and skip the vinegar and then ferment it if you have the time (use a yogurt maker and instructions, then ferment longer for a more sour flavor).