It is kinda hit and miss. I typically find them in the section with the chalkboard and classroom blank stuff. However, they may also be in the kitchen items area. And, sometimes, there is a rack of random slate shapes – hearts, rectangles, etc… – on an endcap.
If you can find an employee, ask them. You might have to go through a few before you get a trail sign.
(Yes, I find trying to find anything in Michael’s to be more of a scavenger hunt than anything else)