They don’t find them all at parties and clubs and bars. There’s a whole raft of material on ‘day game’ - approaches in non-obvious places like bookshops, grocery stores, museums, the high street, etc. which are designed in part to reach women who are unlikely to be encountered in clubs and bars.
Thank you for the correction; that still won’t reach women who don’t get out much in places where they can be easily approached (my gardening/D&D in the basement/all-women gym examples still hold, for instance).
One of the reasons the seduction community has been a topic on less wrong is the application of rationality to success in everyday life. If there is any significant subset of desirable women who are not easily approached then someone in the seduction community will have tried to figure out a way to engineer an approach opportunity. If there are a lot of attractive single gardeners in the world then there is probably a blog somewhere that extols the virtues of garden centres as potentially fruitful pickup venues.
You can argue that the consensus judgement of the community as to what constitutes an attractive/desirable woman is flawed but to the extent that the ‘hard to reach’ women you describe are considered desirable, the likelihood is that someone will have tried to figure out how to reach them effectively.