Hmm, haven't tried the cabinet maker yet, but thanks for the advice.
Well, there is another way to get crossbow bolts for free, which is to rest in an unsafe area of the Broken Tower or the first floor of the Temple (and, I guess, also in the above-ground levels of the Moathouse). If you keep resting you'll generate a random encounter with some brigands, some of whom will be armed with crossbows and bolts. Kill them and then just take their leftover bolts. It's not a clean way to get bolts especially if your party is lower-leveled. But at higher levels, your party shouldn't have any problem in ransacking the brigands without taking damage.