It depends on what sea slugs you want. I have never seen the blue dragon on any order list, and I have very extensive invertebrate lists, and a quick online search showed no retailers there either. You can ask your local fish stores, but it is unlikely you will find one. Many sea slugs are impossible to feed in captivity anyway, some we have no idea what they even eat!
If you want a sea slug tank, get algae eating sea slugs or get ones that you know what they specifically eat and where you can provide that food. Some eat only blue sponge for example. If you can keep a healthy enough colony of blue sponge then they are feasible to keep. Some only eat flatworms. If you can keep a large colony of flatworms going, it too can be feasibly.
Algae is obviously the easiest food to provide. So start with sea hares, lettuce nudibranchs, Chamberlain Nudibranch (really pretty nudibranch), etc.
This place has some nice nudibranchs and information on different species that are more commonly available.
http://www.aquacon.com/seaslugs.html
On sea hares and any nudibranch, take care on your power head and filter intakes as they can easily get caught, and in the case of the sea hare, some can ink or release toxins under stress (nothing more stressful than being torn apart by a pump). Sponge pre-filters on all intakes is recommended.
You will need lots of live rock to provide foraging grounds and natural habitats.
So the main thing is research, research, research. Know what the nudibranch is, what flows it wants, what temps it needs, and what it eats, and make sure you can provide those basics.