I thought I read about this a long time ago, but is there a way to have Netsuite send an email with a URL in it which could click the Approve button on a purchase order? I'm using the custom approval workflow. Would like users not to have to log in and click the button.

You can use an external suitelet that approves that record. You get the external url of the suitelet and put that in your email.

Is that a standard suitelet I can install or do I need to code it?

You will need to code it