Income investors should focus on stocks that pay solid yields, but also those that have sustainable payouts and strong business models.Dividend kings are an excellent place to look for stocks with sustainable dividends. These are companies that have raised their dividends each year for more than 50 consecutive years.The following three dividend kings have low dividend payout ratios and recession-proof business models, meaning their dividends are well-covered even when the economy is suffering.Walmart (WMT)S ...