Rune-carved tablets are a pain to find in bulk, of course. I was doing some Moria crafting instances today and the Mekeb-fahem instance has quite a few of the Sage's Locker, the rune-carved tablet dropping nodes. That's one of the ones in the winding way between 21st and Chamber of the Crossroads. I got nearly 40 RC's out of it. The other instance has Sage's Caskets, which drop tablets too, though much less often: I got about 10 from that instance.

Also, you can trade 6 of the resource tokens you get for finishing quests in there for a "Collection of Lesser Khazad Texts" which were dropping another 5 rune-carved tablets for me, along with other Supreme scholar mats. The other option is a key for an Iron Garrison Resource Cache, which will also give you some tablets along with an assortment of other Supreme level crafting stuff for all crafting professions. Or, if you're more interested in Loth barter stuff, you can get a golden leaf instead of either of those.

These instances are a breeze if you're over-leveled, and though you will get no XP for the quests if they are trivial (grey ring), they still give LIXP and Iron Garrison rep as well.