Like you, I love to talk about whatever special interest I have at the time. Like you, I found it extremely rare to find anyone that interested in the topic to actually "light up" with excitement because they too, have the same interest. Most people, in my experience, are interested in other people, not things. Even neurotypicals that are truly knowledgable experts in their field, and can elaborate with great detail when prompted, still tend to focus more upon their interpersonal relationships during everyday conversations.
I don't know about you, but I have made the mistake of trying to make the conversation transition into some aspect of my special interest,...and then begin an educational monologue on the topic. Most, if not all, people don't appreciate that. They will back out of a conversation as quickly and as politely possible.
Now, as you mentioned, the most logical solution to this would be to join a community, organization, or club with the same special interest as you. However, this often presents another potential barrier for the autistic individual,...to be an "active member", you have to actually participate, even meet the group from time to time. So, I have in the past been "active" in powerlifting organizations and meets, autocross and open track events, respiratory care conferences and organizational meetings, etc. Then there are the other special interests that I have that I have enjoyed myself, freshwater aquariums, bonsai, orchids, aroids, caudiciform plants, rockhounding for various fossils and minerals, etc.