Dave de Vos
Do I understand correctly that good internal strength teachers are so rare that seminars are about the only option for learning what it is?
You can get lots of information online. Ask your favorite search engine for "internal strength and martial arts" and you'll find plenty of stuff.
But that kind of head knowledge won't really do you much good. It won't help you understand the difference in feeling between aikido techniques done with and without aiki, and it won't help you actually develop internal strength on your own.