Could be about the demand. I would think that if you are going to pay somebody for fixing your problem the cheapest solution would be a new wheel.

If wery few peopel can fix such things at home the demand is not enough to justify selling them and if peopel do not know where to start to look for the cones it`ll look like the demand is low...

I suggest the OP email Brompton mothership and ask. Maybe they can give you a parts number that you can then give to your dealer- or it is not a Brompton specific part so they could tell you where to buy it.

What about e-bay?

Edit: https://www.google.no/search?q=bicyc...ts.+axle+cones