Have you tried her summer home at Monster Island?

Seriously, to the best of my knowledge only ScottA ever released a Mothra figure for Poser, and I don't think he finished the textures on his. You'd have to ask him; I don't have a copy.

Traveler has some nice, detailed butterflies for sale over at RunTime DNA, however, if you feel like doing a paint- and morph-job yourself. Or were you looking for Mothra's immature caterpillar form instead?