There are many, many places...
There are many, many places to look online. On this site alone, there are dozens of listed vintage dealers. Just navigate this site, there is more to it than just a forum.
As far as marks, a sticker is always nice, but more often than not they have fallen off. A Herman Miller Shell will have the embossed logo underneath.
You should try searching on the supplied site search engine at the top right. There has been a lot of talk on this subject. Good luck.
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