The primary source of prices for the iOS and Android wallets is currently Dash Retail (which themselves get data from 12 sources). We also have fallback sources in case of service interruption; CryptoCompare, BitPay, DashCentral, Poloniex, LocalBitcoins and BitcoinAverage.com. We have recently learned that BitcoinAverage.com is changing its service plans so we are in the process of looking for alternatives for this particular fallback source if needed.