I think the short answer is that shares, or indeed whole planes, are not (in the UK) advertised in any one place.
And by the time something has been out for long enough to get picked up by google, there is probably a reason why it has not sold.
A lot of shares change hands informally, without advertising.
If I was looking for one (as I was a few years ago) then I would contact the administrator of each local syndicate (operating something interesting) to see if any shares are up for sale.