Looking for a gartel that goes under the shirt to keep tzitzis straight but have only found the large ones that married men wear. where would one find the smaller thinner kind that can go under a shirt?
Try getting a piece of elastic at a fabric store. Get a long enough piece that you don't have to stretch it too far.
You could also do what some chassidim do for their regular gartlech and use a couple shoelaces tied together (without the aglets).
you can get them from Judaica World. Here is a link to order from... http://www.judaica-world.com/elastic-bendel-belt.html