Prong Set Five Opal Septum Clicker
1/4" clickers inner diameter measures 10mm wide x 6mm tall
5/16" clickers inner diameter measures 10mm wide x 7.5mm tall
Most of these are tough to open and close at first. You will probably have to adjust the tightness of the clicker with ring opening pliers.
More septum clickers are available here.
Jewelry is measured through a gauge system (see chart below). The higher the gauge number the smaller the wire is. A standard "earring" is usually 20 gauge. Jewelry gauges/sizes can vary between manufacturers. There is no universal regulation on companies to make a standard size in body jewelry. A big part of this is that some jewelry is made overseas and measured in millimeters. And in the USA jewelry is measured in inches/gauges. So many times jewelry can be anywhere from .5mm to 1mm different.
5/16" (~8mm) diameter 3/8" (~10mm) diameter 1/2" (~12mm diameter)