Hi AlanI too wear thongs under my tights.  I like the fit better than a dance belt.  Try Metromanusa.com, it's great company here in Seattle that has a HOT webpage and sells great undergear.  Go to the THONGS page.  You said you were looking for a nylon thong which they have but I love the one called ROCKY.  Its cotton/lycra but doesn't have a seam in the pouch which is great under the tights.  And the fit is great, nice and snug and well made.  I have black, white and grey depending on the tights I might be wearing.  I wear them always under my lycra shorts and tights at the gym.  Good luck.  Rob