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It is a common misperception about potassium that bananas are the only food that supplies noteworthy amounts of potassium. In fact, there are several food items that pack more potassium per gram than bananas. Just 3 Medjool dates already give you more potassium than 1 banana. Other very good sources are avocados, dried apricots, tomatoes, potatoes (as you mentioned), Swiss chard, yoghurt, …

The best source to find nutrient content is the USDA database (https://fdc.nal.usda.gov/food-search?type=Foundation). You can search by food item or by nutrient.

The reason why you don't find much K in multivitamins is bulk. K is a macromineral. A day's supply of 4 grams wouldn't fit into the regular tablet size. Moreover, such high amounts in a supplement may be contraindicated in people with compromised kidney function or on certain medications.