Transactional Templates within your SendGrid dashboard are located at https://sendgrid.com/templates. There you can create your own email template. They have decent documentation to help you get a template setup, however they don't provide any default templates to choose from to help you get started.
There are many places where you can purchase email templates that can be used, just copy and paste the HTML that you get into the HTML editor in SendGrid. A simple Goolge search for "Email Templates" will return thousands of potential options.
Email on Acid has some nice choices, many are free.
Theme Forest also has many options, all of which cost money however there's better quality with most paid templates. Find a style you like and check out the reviews and decide what works best for you.
It's best to avoid templates designed specifically for services like MailChimp or ConstantContact because they typically include code specific for those services which may cause problems for you in SendGrid.
Be sure you insert the <%body%> tag into the template where you want the email content to go. Without that tag, the emails will arrive to the users inbox without any content or broken.
Once you have created and saved your template, copy the template ID which we'll need later when we finish configuring your sit to use the template you just created.