Where do I find a sign up form
 

I would like my clients to sign up before they use my IDX propert search. How do I do that?

If your IDX has a contact application built in, then that's the easiest way. If they don't you'll have to setup a contact form within the site. You can download a free one from the WordPress plugins page. However, this is something that would require some customization and coding, and is outside typical theme support. I'd recommend reading about the available contact forms in the plugin repository and see which suits your needs. I also recommend Gravity Forms.