One Magical, Flying Kingdom's Trash...

As with all drug dealers, the first one is free.

[npc=67565] is occasionally present at Halfhill. On days when he is he'll be standing facing a table between [npc=58716] and [npc=64395]. Testing on various characters at various rep levels suggests that you will not be able to get the quest, nor buy the shards until you are Revered with the Tillers. (Much like Songbell seeds)

At revered or higher [npc=58718] will give you the quest A Shabby New Face asking you to speak with the visiting Mage.

Completing One Magical, Flying Kingdom's Trash... with [npc=67565] awards an [item=91806] which you can plant on your farm in Halfhill to produce the following...

[item=91850]
[item=91860]
[item=91861]
[item=91862]
[item=91863]
[item=91864]
[item=91865]
[item=91866]

You will, of course, only be able to grow portals for your own faction.

Every shard you plant will take the normal amount of time to grow, just like other seeds. Each one will need to be clicked on to stabilise it resulting in a [npc=67543] appearing, once defeated the shard will be able to grow.

Quoted from 5.1 patch notes

•A new Seed has been added: Portal Shards
•A mage who fell on hard times back in Dalaran has found a new home on Pandaria. [npc=67565] will occasionally visit Halfhill Market to peddle a new type of seed – and take advantage of the Tillers’ hospitality.
•Portal Shards grow into consumable items which can be used to open a portal from Sunsong Ranch to a major faction city.

Note the occasionally. You'll want to stock up on [item=91806]s when [npc=67565] shows up to cover you for days when he doesn't, since [npc=58718] doesn't sell this type of 'seed'.

...is a magical, mist-shrouded kingdom's trash, as well.