There are a few foil membrane type insulation products out there.
One is a foil bubble wrap as Matt mentioned.
Others are known as "Tri Iso"... Tri Iso Super 10 is the standard these days, but you might be able to get some Super 9 (no longer used so much in things like loft conversions thanks to changing regs) for a bit cheaper.
There's another called Thinsulex made by Web Dynamics which is similar.
Advantages of any of the above are flexibility (dunno if your garage door is a single piece up and over or a segmented one).
Also, it'd be quite easy to cut small openings in it to allow you to get to the door hardware from the inside.
EDIT
Another option is one some people use when they are making their own camper vans from commercial vehicles...
Expanding foam spread onto the door and allowed to expand would also give a reasonable level of insulation.
A bit of time taken to box in things like the door handle, cables and the likes would leave everything working as it should and easy to access.