As eggs. Like sea turtles you study the biology of the creatures including environmental factors that may effect hatching. You then study means of placing freshly fertilized eggs in a torpor like state so they can be kept in storage until needed.
You also use selective breeding so that the only eggs being produced come from dragons demonstrating the characteristics you want. So in this case only the approachable/ reasonable/friendly dragons get to produce eggs. Some eggs you raise for more dragons and some you put in storage.
As a by-product any eggs that pass their use by date? - giant omelettes.