Find the theatrical supply houses in your area. They will be listed in the phone book--either the physical book or online version--under "Theatrical Supplies." If your area does not have any, locate one in the nearest major city. Music stores that provide performance gear for rock musicians and DJs may also be good resources.
Measure the width of the gel frame holder on the lighting instrument you want to use as a moving head fixture. This is the slot at the front end of the instrument where color filters are placed. It may be a grooved square or a set of clips that are attached to the front of the light. Standard widths are 6 inches, 7.5 inches and 8 inches. Many theatrical supply houses rent and sell drop-in attachments that fit in these slots and contain motorized mirrors. The mirrors are cheaper than fully automated fixtures, but they may only be available for instruments with true 6-inch gel slots, such as ETC Source Four or Altman Shakespeare ellipsoidal reflector spots.
Contact the theatrical supply houses and music stores you found and inquire about purchase and rental rates for the drop-in mirror attachments and rental rates for moving yoke fixtures. Also ask if they sell off any used rental equipment, and whether they have any working moving-head fixtures available at a reduced rate.
Measure the distance between where you will be hanging the moving-head light and your lighting control console or nearest DMX port. DMX is the type of signal that most modern light boards and moving light apparatuses use, and you will need to purchase or rent enough cable to reach from the apparatus to your light board or port, whether you are using a motorized mirror or used moving light.
Consider pooling resources with other local performance groups to purchase one of these units and share it among you. If you rent out a performance space, ask that space for the contact information of other groups who also use the space. Since you all share the same venue, it is unlikely that there will be conflict regarding when each group needs the shared item, as you won't be booked in the same space at the same time.
Compare the purchase price to the rental cost of each item, taking into consideration the cost of the necessary amount of DMX cable and whether you think you might use these units in the future--for one-time use, rental is best. Decide on the most cost-effective solution and arrange the rental or purchase.