I've noticed people in the trade districts offering them fairly often (~ once an hour, if not more frequently) for 7-8k. I saw one wts ad for 4k and another for 5k, but those seem rare.
I've noticed this while standing in trade districts watching the scroll for a few hours looking for something else.
If you need one, you could go to Kamadan and put up a wtb ad in the party search.
And the post before is right - ANet didn't remove them. Unlike merchants, who have an unlimited supply of things like id kits, the rune/material/dye traders get their stock from players. If more players are buying an item from the trader than selling it, price goes up. Eventually, the trader runs out of stock.
Every once in a while you will see the rune in question appear on the trader's list again. It happens after someone, somewhere sells them one of the runes but before another player buys it. Some people will recommend you just camp the trader, checking stock over and over. I've no idea how long it takes to get one that way.