In general, any question related to where one might buy equipment or software (retro or otherwise) or asking for buying recommendations are usually off-topic across all SE sites. As stated in other answers, the answers for such questions will get stale fast, and will need constant grooming. Not to mention that list-type answers will have to be organized into general- and system- specific sections.
No one really wants to maintain such a FAQ-style document, and SE doesn't really want us to.
That being said, Retro is a bit different, in that where to find things is invariably part of the hobby.
We might want to check how Amateur Radio SE handles these, as digging around for old equipment is a shared love between us. I suspect the answer will be "your local meetups and clubs can point you at online vendors and swap-meets" but I am happy to be surprised.
There is talk on this Meta about a blog. This might be a good subject for someone who wanted to maintain a series of blog entries in a FAQ-style manner for finding old equipment online.