Some of the merchants at Valua are hostile. I believe the enemy formation is always the same per individual merchant, meaning some of them will always have Valuan Laelaps with them. They revive eventually, so you can go back to fight them again periodically. Have Jyla use Collar Hack to trivialize them (they'll waste most of their turns trying to activate her nonfunctional collar), and then spam Steal on the Laelaps.
The one I usually use for this, is to the left of the warp point. One Crooked Trader and two Valuan Laelaps. Maybe even bring the Commoner in the active party with Dumb Luck learned, so you get higher drop rates. Or have Jyla be a Masked Noble and bring some Faceless Maids along so you can have multiple members spamming Steal together and rob the enemy totally blind. ...I wish I'd remembered that concept like two days ago.
As a bonus, the Crooked Trader has a high money yield but unimpressive EXP yield (250G but only 7EXP), making her a prime target for Wind's Bad End Bullets and Dark Dealings. Valua is a fantastic place to get a lot of money in only a few battles this way. (The Laelaps have even more money, but 17 EXP each is kind of a lot by this game's standards.)