Ranch Products moon clips are excellent and least expensive - ca. $75 per 100. As a previous poster mentioned the grooves in 38 Special & 357 Brass vary greatly. With Ranch Products moonclips I find W-W nickel plated brass gives the tightest workable fit but their moonclips are also very useable with both R-P and Federal brass or nickel. Starline brass has a very loose fit in Ranch Products moonclips and would be unacceptable for competition use.
I hate paying for expensive moonclips which are trampled by fellow shooters at competition shoots. With Ranch Produts moonclips I liomit my loss to 75 cents per clip.