4e doesn't assume that everything that's done in the game by NPCs can be done by the players via the existing game rules.
There may be rules sometime, but for right now - they're there for the same reason the towns are there, or the dungeons are there. Someone put them there.
Revel in this freedom. You're the DM, so make the most out of this. If your players ask, "How do we make one?" ... that's your cue that they have a goal, and it's your turn to make something up. Heck, make an adventure out of it - say, the knowledge died with the Empire of Bael Turath, and it's up for the PCs to find it buried in their lost catacombs. Or tell them it's a 30th-level ritual and to talk to you in a year. Or that it takes weeks of work and the secrets are jealously guarded, but that a nearby sage could be persuaded to do it for a price.
The options are limitless.
-O