How rare are these and what does one have to do to get them?

It's like there is an entire category of dye I cannot obtain, except for the 3 metallic colors on the MogStation. I know that they can be obtained by retainers on ventures, but my retainers must be terribly bad at finding dye, since I've had none. I was hoping that these might be used as prize items at the Gold Saucer, but that has come and gone without the dyes appearing. So, how do you get them?