Mandrake Cordial and Cherry Cordial is an ingredient for the White Gull recipe. Later on the game, you’ll be needing White Gull as it is used for crafting higher level potions and gear. Mandrake and Cherry Cordial doesn’t seem to be looted in the game, however they can be bought from a merchant.
The merchant that sells them is in Crows Perch in Velen. The top merchant on Crows Perch near the armorer also sells it.
The merchants only have 5 bottles of each when you open his stock. However you can immediately replenish them by simply exiting the merchant conversation and talking to the merchant again.
Update: Seems you only need to close the merchant inventory stock and talk to him again to replenish the the items (no need to exit the conversation).
Update: Recent updates made the closing and opening of the merchant inventory page not refresh their stock anymore. On some merchants (like in Harviken Skellige), you can medidate outside the pub to refresh the merchants inventory. Sometimes, saving and loading your instance also refreshes their inventory.
A much better location (more update below; Hierarch Square).
Here is a better merchant in Velen which has a much shorter travel distance than the one on Crows Perch. He is located in the Inn at the Crossroads waypoint.
The great thing about this location is once you waypointed, you can walk straight inside the Inn.
This merchant also sells Redanian Herbal. Redanian Herbal is also required in the White Gull recipe. Although it can be looted throughout the game, in cases where you ran out of it, you can always go back to this merchant.
Remember to refresh the merchant’s stock quickly, just close the merchant’s inventory window and select the “show wares” dialogue option again.
There is also a merchant in Skellige Isles that sells both Mandrake cordial and Cherry Cordial. He is an Inkeep and is located in Harviken which is South East of the map.
When you waypoint on the marker, the Innkeep icon might not show yet on your minimap. Just go up, passing the grindstone/blacksmith and the Innkeep icon should show up.
If the icon wont show up,give it a few seconds. You can also go in and out of the Inn and wait for the Innkeep mechant to pop up.
Update (10/20/2015) : higher stacks of Cherry/Mandrake Cordial:
In recent updates (not sure if available to HoS players only), there is a new(?) merchant in the middle of Hierarch Square. He sells Mandrake and Cherry Cordial in great stacks. This is a nice addition since refreshing the merchants inventory page by closing/opening the dialogue doesn’t work anymore.
Where To Find White Gull
If you haven’t got the White Gull manuscript yet, you can buy it from the White Orchard herbalist. See below, its even available during the early part of the game (am level 4 on screenshot).
This herbalist also sells the Troll Decoction (although covered above).
White Gull is required by all potions when you upgrade them to the Superior type and is required by their sub-ingredients. You can view the list of potions in the game here.