;^) Heh. Just be glad that you can buy undead buster. If I recall correctly, mimic busters cannot be purchased anywhere, so you must obtain them by defeating mimics, instead. Usually bombs work best for this, and the Sun & Moon Temple is the best place to find lots of mimics.
:^/ As an aside, the items available in Suzy's Watery depend upon the store name you recommend to her. Try each of the possible names (resetting without saving each time) to see which inventory you prefer before saving the game and moving on.
;^) Of course, that advice comes a little late this time, but next time you play through Dark Cloud again....