They are getting very tough to find. Have you tried www.culttvman.com? Also lurk around www.starshipmodeler.com, you may get lucky. Monogram produced four Voyager kits. The original one was a decent kit, they also produced a limited edition one with some clear parts, shuttle bay and superb decal sheet. The third one was part of a 3 ship set with the Kazon and Maquis ships and the last one was this tiny little pre-painted snap-tite kit. Revell of Germany also boxed the original Voyager kit.
Best bet is the original kit, but they go for several times their original $20 shelf price. The limited edition kit went for $30ish at first, they most likely go for a king's ransom.