James,
Thanks for your pm, I don't normally use that function, so in the interest of David's site and in particular of his forum, I'll be happy to talk to you here.
Generally speaking, if you want to buy a particular medal or group and you feel that it is likely to exist (I always tend to do so, unless reasonable proof is available to suggest it does not!) you can join the British Medal Forum and place a "wanted" advertisement with them, also do the same with Medal News.
Whilst that is something I have never done myself, I would say to you, just go and do it, in fact, I'd say do it here as well and as many other places as possible.
Then, have a look at the London Salerooms archives to see if they have every turned up there, if a particular medal has been owned or is in the current procession of a person other than the recipients family, the chances are that it will been offered of sale in public.
Medal collectors can be very odd people and as mentioned before, are often very private too, with some types of medals, the QSA is certainly one, despite, the vast majority being issued well over one hundred years ago, there are some that although offered for sale, have never actually been in the open market place.
For example, there used to be a fellow in South Africa who had actually obtained much of his collection from the actual recipients or their immediate next of kin, a little after the Second World War, he sold or traded particular examples in the 1970's with a couple of collectors here in Great Britain, some of those same were sold on and bought, now they reside in a particular collection here, yet, they have never really been on the market for nearly four generations.
So you see the potential problems that a person can have when looking for a particular recipient.
On the other hand, on occasion, you might be very lucky indeed, as a boy, I identified a number of certain RFC/RNAS/RAF recipients whose medals I knew nothing much about, but, then as an adult, I was able to find out the all important why, if, as and when particular medals were actually issued and awarded to the same.
These medals have been and continue to be a absolute joy to collect and I never give up looking for certain recipients that I've spent almost a lifetime looking at, so I wish you bon chance and I hope that you find what you are looking for, it's so often the case of just being in the right place at the right time, keep your ear close to the ground, again, best of luck to you,
Kind regards Frank