You won't always be able to find DOIs for electronic resources. However, handles provide more stable URLs for some electronic resources that don't have DOIs. For instance, many institutional repositiories use either handles or DOIs.
What is a handle? According to the Corporation for National Research Initiatives (CNRI), "The Handle System® is a component of the Digital Object Architecture and provides efficient, extensible, and secure resolution services for unique and persistent identifiers of digital objects."
There are a wide variety of resources that might have handles including:
Corporation for National Research Initiatives. (n.d.). Welcome to Handle.net. Retrieved from http://www.handle.net/
Depending on the website, the place to find the DOI, handle, or permalink might differ. If you see the string "hdl.handle.net" in the URL for the resource's record, it is a handle. If you aren't sure, copy and paste the handle's entire URL into your web browser to try it out. The handle's URL should link you back to the resource.
The example below is from a record in DRUM, the institutional repository for University of Maryland, College Park.
NOTE: You should still verify that the resource doesn't have a DOI before listing a handle in its reference.