You can make them yourself by getting a bottle of childrens powdered water color paint. Mix it w/ water to the consistency of a paste and coat the underside inside of the clay. Let it dry and you have a flash target. It adds a little weight, but not enough to make a real difference.
The reason for Flash Targets are so that the televised finals look more spectacular and this is one way of getting people interested, if you see it in action at a final you will be quite impressed,they are a little heavier as the underside has a pocket of powder in it that gives the flash effect.As our club hosts the Australian ISSF nationals we use them only in finals as they are expensive.
How many times have you walked out on the field and seen whole targets lying on the ground that show several pellet hits but never broke??
It is possible to have a pellet hit a flash target and break the powder and score a hit but the target never breaks.
This is a rule variation unique to flash targets.
The normal rule is a visible piece is required to register a dead target. But when you use flash tagets the "dead or hit" requirement is changed to require only powder be visible.
You can make them yourself using talcum and newspaper glued under the dome. Be a fun project for some crappy rainy day. The weight change is not material enough to alter flight characteristics (at least not in international trap or skeet).
About a year ago, I talked to a Josh Taylor who was/is the eastern sales representative for White Flyer which is a division of Reagent Chemical and Research. I was asking him about the White Flyer flash target and their availability. He did indicate that they were costly perhaps about 4x the regular target. He said that you can make your own if you want a few for special effects. His suggestion was to fill the depression on top of a regular target with a powder and then put a piece of tape over it. I have done this and it works very well. I have used flour, chalk line powder, and paint powder. I used 3" wide masking tape to cover the powder--a 3" long piece is about right. I don't know if these would work in automatic traps but they do work well in my Lincoln 3/4 cock traps. These targets add a little wrinkle for fun and games. There have been some previous posts on this forum relating to the same. Hope it works for you if you try it.
We made these years ago by putting poster paint powder in the top, laying a bead of white glue on the rim, then laying a circle of light paper on top, tamping it down for a secure fit, then putting them to dry. They load the same as any other target in our machines. If you are having a game day, you can put one at different levels in each cylinder of the carousel and if it is broken by a shooter, they get a special reward. This can be a beer at the end of the days shooting, double the prize you are shooting for, or whatever else you can come up with.