I see that Raileasy sell them on line in theory (drop down for origin/destination 'London International CIV') http://www.raileasy.co.uk/
Alas raileasy no longer seem to sell them. I last bought one a couple of weeks back for Newcastle-Kings Cross to connect to the Eurostar, but since they've changed their interface to automatically serve up their split ticketing approach they won't quote any fares to either London Eurostar (CIV) or London International (for Eurostar) either there, or on their 'classic' site which is now https://trainscanbecheaper.info. I've queried with them whether this was intentional (after all, they are showing the CIV options in their typeahead search) but no response yet.
Yes, they are still a thing. However it is getting much harder to obtain them. You may have to try a booking office.
Given the above you would appear to be right!