I need a simple way of dimming 12V Halogens

I work with a preformance arts company doing shows in the scottish
highlands and due to the massive amount of rain fall we need to use
water-prof lighting.

We have been using 12v hallogens with some success but I cant find any
pre-built 12v dimmers. I have found several that use 240v dimmers and
then drop the voltage down to 12v but none so far that start with a 12v
input.

My limited understanding of switch mode power suplys makes them seem
like the logical solution but  I dont know enough to pick the right
chip.

I have found some that require only  couple of external componets and
only a small heat sink.

If someone could point me in the right direction it would save me a
great deal of heart ache.

Thank you for your time.
Nik