Generally only way to get them is via completing an "arduous selection course" ie P Company, Commando Course, Hills Phase, Raf Regt Pre Para and I think possibly the Army Special Observer course.
Plus you must be posted to a unit with an airborne role. You used to be able to do the course and get the lightbulb (this was if you were'nt going to an airborne roled unit, you would still have done one of the above) but it would be practically impossible to get on the course if there was no operational requirement, unless of course, you were going to be King.
There are a few randon RAF blokes - TSW/TCW that get on the course as well as every now and then some specialist will convince HQ 16AA that they just have to have them because they'd be so valuable etc. They won't have done a selection course.