dear element109,
no matter what you buy - MoO3 or (NH4)2MoO4 - you still should reduce it first - at least, molybdate won't react w/acid either. Just combine some molybdate w/Na2SO3 (or some such), slowly add acid, then basify completely, filter the precip'd Mo(OH)2 (i don't know exact Mo's valency here), wash w/water, dissolve in HCl, evap liquid.
Antoncho