Most distributors will carry a clear flake. It comes in several different sizes. My favorite (if I remember right) is the .08 micron (a medium flake size).
Best way to apply the clear flake is as follows.
Create a separate film wherever you want the flake to appear. If you want it on top of a specific color only, either make a second film or shoot that film twice. A 156 mesh should be fine. You will be printing a clear ink through this screen. Each ink manufacturer makes a clear ink with extra adhesive properties, it is that ink you will want to use.
Print your design as normal, flashing your design before printing the adhesive clear ink. At this point, remove the shirt from the platten while the clear ink is still wet.
Here's where you have several options. Most people will have a tray that they have poured the flake into. Carefully take the shirt and place it wet print down in the smooth tray of flake. Lightly press on the back of the shirt. Carefully pick the shirt back up and shake excess flake off. Cure as normal for the print thickness you have on the shirt. Conversely, instead of a tray, you can use a Parmesian cheese shaker to shake the flake onto the wet portion of the design, shaking the excess off into a tray to recycle. And yes, it has the potential to get messy. There's no wrong way to apply the flake, If you can think of another way, try it
If you are on an auto there is another way to do this as well.
Once the shirt is cured, it has the retail look that has been popular for several years, making the ink look like it's a glitter.
You can also use the same technique with any glitter of choice for different effects.
Now if your customer wants it to definitely be colored glitter, then the search will continue for the flake color you want.