Where is explosive decoy schematic?

I found this last time, and the Prima guide says it is sold by the inventors (Tarant, Black Root, and Wheel Clan), yet even with the shopkeeper inventory hack (makes all their items available all the time) - none of the merchants (I have also tried the blacksmiths, herbalists, gunsmiths, and magic stores) have a copy of the schematic for sale.

Where is the explosive decoy schematic? I am beginning to suspect it was a schematic I found last time, rather than bought, but I can't even begin to guess where.

Help please.

Thanks.

 

I bought it from a gun technologist in Tarrant.

 

I tried him - and he is supposed to have it - but he does not.

 

The gunsmith in Ashbury (if you keep going back) will sell you just about every gun, bomb, and grenade schematic on the market.

Buy any new schematics you see, then cross the street rest till morning. Repeat till you have them all, run out of money, or get sick of sitting there.