Hi Andrew,
Looking at the example you were following, I pulled the code number off the picture of the relay. When I typed that code number into Google, the first thing that popped up was its page on Mouser: http://www.mouser.com/ProductDetail/Omron-Automation-and-Safety/G3MB-202PL-DC12/?qs=ISjK4nnpKfGtUJvc.... As you can see from this page, the reason you were having trouble following the link is that this relay has been discontinued, and is no longer available.
If you scroll down on the Mouser page, however, to where the Specifications for the device are listed, you can choose which characteristics of the device are important for your application using the checkboxes to the right, and then click the “Show Similar” button at the bottom of the page. This should take you to a listing of all similar parts that Mouser carries, and should give you a good place to start looking for substitutable components.
Let me know if you have any questions, and I’ll do my best to help you out!
Kathryn K.
Technical Support Engineer
National Instruments
http://ni.com/support