When looking to invest in domains, you have to identify keywords which are searched online. Those keywords can then be expanded into domain names.
For instance, if I want to create a portfolio of car related domains, I may look for car.com, usedcar.com, cardealer.com, buycar.com, etc…
The best way to find those domains is to use a keyword research tool. You can use of of the free tools around but I would strongly advise to save time and use a tool designed for it: Keyword Discovery can indeed provide you with lots of suggestions but it will also check the availability of domain names based on those terms, saving you lots of time.
And as they say: “time is money”. Have a look at a few screenshots: http://www.keyworddiscovery.com/domain-research-tool.html