Try to find a state or local goat breeders club. Or maybe even contact your extension office. If that fails:
You can choose a neighbor, if you have one willing, then there's a test they have to take. If they pass it, they can be your testor. They have to come to your house at least every 45 days to do test, which involves coming for morning milking, recording weights, taking samples, and for evening milking, recording weights and taking samples again. Then your testor is responsible for sending the samples. Most people do it once a month because it's just easier and the data is more accurate.
You can do your own testing, they have a program that's called "owner/sampler" but your animals won't be eligible for the AR (advanced registry) if you do your own tests, and since that's why you want to do the test, it wouldn't make sense for you to go through that and not get the ar designation.