This comes in two varieties.
This requires a monthly subscription and allows you to retrieve data on the latest trades. To get this data via the Stocks API, you must subscribe to Yahoo's real-time service (Yahoo charges a small monthly fee for this.)
To check the latest prices and/or to sign-up, please visit the Yahoo Finace Real Time Signup page.
This is made available free at present from Yahoo Finance. It provides the data with a delay, which can be up to 20 minutes or more. Any subscription purchase is not required.
TD Ameritrade provides realtime streaming trading data for its clients. If you have a TD Ameritrade account, you can use it to retrieve data programmatically via the Stocks API. TD Ameritrade is a broker, therefore you can also place your trades with them using the Stocks API.
To sign up with TD Ameritrade for an account, please visit the TD Ameritrade website.
NxCore is a high-end realtime trading-data-source provided by NaNex. This is a relatively expensive but full-service source, and you can also purchase historical data from NaNex.
To purchase or subscribe to trading data using NxCore, please visit the NaNex website.
Juno Trade provides realtime trading data as well as trading services. A monthly subscription to Juno Trade API must be purchased and maintained.
Desiderata is working with other brokers/data-sources as well, but at present, you will need an account/subscription with at least one of the above in order to be able to get realtime quotes using the Stocks API.