I know using templates and stencils is not the preferred way to do a traditional zentangle. But ultimately the goal is to find the place that gives you your own zen - and since there are no rules in zentangle, this works for me. I don't always use a template - I like the free flowing ones that I sometimes get by hand too. But if I'm feeling stuck, a template can start my creative juices flowing. Below are a collection of templates that I have found useful. They include the usual ones you find in art supply stores but also scrap-booking supplies and even a medical french catheter scale. (I have no recollection of where I found that one!) Not pictured are a collection of clear plates in various sizes and rolls of tape that I use to get circle shapes from too.
The one rule I have for myself however, is that I never use my templates with a pen. I always use a pencil with a template and then do the pen work by hand. I feel that this give my work a more authentic feel.
This is a french curve. There are other sizes and shapes also.