Frozen semen sales are handled by Reproductive Enterprises. They sell frozen semen for us and other breeders. Their link is below. The link is to the Nubian dairy Goats on their site, not just my goats. If you search their site, you can find other breeds of interest. My experience with them is that they are very professional.
If you are new to breeding goats via artificial insemination, then I recommend Langston University. They usually have an annual AI clinic in the fall. I was very impressed with their class. They also offer other goat related educational classes.