You can buy Emerald Dust from that very same Quartermaster but I wouldn't advise it since it's really expensive. You can easily get Emerald Dust from killing Wraiths.
In White Orchard there are a total of 4 Wraiths.
Since you say you have already the diagrams for the swords I assume you've already fought the 2 lvl.7 Wraiths in the Cemetery (One by the Place of Power and one inside the Crypt). If they don't drop Emerald Dust you still have 2 wraiths left.
One you can kill doing the quest "Devil By the Well". Do it and loot the Emerald Dust from the Wraith.
For the last Wraith it's actually close to the Nilfgaard Garrison and it's in a Place of Power. Go there and a Wraith will appear to protect it. Kill it and retrieve the Emerald Dust.
You can use this as an interactive map and in case the link goes dead here is the location:
I did this and it worked like a charm.
Each sword only needs 1 Emerald Dust so you should be good to go if you have the other ingredients.