There are a few brands of tofu or soy-based "sour creams" on the market, touted as dairy-free and quite tasty. Keep an eye out for Better Than Sour Cream and the Vegan line by Galaxy Nutritional Foods, both widely distributed brands. Of course, in a pinch, you can substitute sour cream with any of these ideas:
Ingredients, processes and products are subject to change by the manufacturer at any time. All products should be considered at risk for milk cross-contamination. Always contact the manufacturer prior to consumption.