The default provider is Yahoo for new stocks. However, a different provider can be defined for each different stocks with the Stocks Panel. Google, Quandl have each their advantages, like currencies for Quandl. Also, local providers like kbd_Japan for Japan and India_NSE for India are focused on local exchanges.

Intraday data are downloaded from
Google or Yahoo, for the most part.

You also have the ability to use Paid Providers to access extra data. At this point, EODData (Platinum account) offers access to commodities, futures and foreign exchanges markets in the US, UK and Canada. Quandl also offers Premium datasets on top of Free datasets. You can access both.

Stocks are updated when you are about to display them or, if you have open trades, when you open the investment panel.

Yahoo: Yahoo has the largest stock coverage and is, by far, the fastest. The search field, in the stock panel.

Google: Google only covers US, UK, China (inc. Hong Kong) for historical data, but it is the provider for most of the intraday data.

Quand: Quandl is an open collaborative effort with millions of data sets from hundreds of sources, either free or premium. You have to do some research but it’s worth it. For example, code for Apple is GOOG/NASDAQ_AAPL, code for Gold is OFDP/GOLD_2, etc... Note Quandl covers many other things beyond stocks like real estate data, crime trends, or even UFO sightings (NUFORC/SIGHTINGS).
Important: Registration is free BUT you have to register before using them. Once registered, go to your account profile, API tab. You’ll see a 20 characters authentification token. Enter that token in Mastock, Preferences, Internet page.

GoogleQuandl: Designed as an alternative to both Quandl and Google, it accepts either Google or Quandl codes. The reasons this provider was developed are:
  • Google enables long term historical data only for the US, UK, China and Hong Kong markets. This provider extends coverage to all Google codes, all markets.
  • Quandl requires free registration.
The key limitations however are, (a) only the last 200 days are initially downloaded, enough for most indicators and (b) it depends on Google good will not to change key construction of their download web pages. If issue, just select all stocks with this provider, and switch to Quandl all at once.

EODData: You need a Platinum account to use them and, as of 2013, it’s 29.99USD/month. They cover many exchanges, including Futures, Commodities and Currencies. Once you have your account, go to Mastock, Preferences, Internet and enter your user name and password.
To add an EODData code, you then have to use the
Third-Parties Screen

KBD_Japan: Covers several japanese stock markets like Tokyo, Osaka,... Tends to be slower than other providers.

India_NSE: Data directly from NSE.