Answer:
Regardless of what genre you are interested there are but a few fundamental ways to obtain autographs. Each has their own set of risks but one;
In-Person; at trade shows or as an autograph hound is the only way to ensure you have an "authentic" autograph. Take some pictures of the signing and any other provenance you can acquire to record the date, place, event and so on for posterity later.
TTM; Through the mail if you can acquire accurate mailing lists. The websites of many places also provide addresses. However, going this route can also lead to many secretarials, pre-prints, auto-pens and fascimile signatures. Just do to the sheer volume. Some mailing lists claim accuracy but are woefully inaccurate.
Buying them from a reputable seller, dealer, or auction house. Each have their plusses & minues with regards to "authentic". Check the T&Cs and even though someone says "it's guaranteed for life" it implies the life of the place where you bought it from. Some over "industry" TPA (third party authenticators) but while they have a good positive track record they can and have made some blistering errors.