There are plenty to chose from... look here
Battery Cut-Off Switch - Google Product Search
I'd recommend this one...
Pegasus - Master Battery Cutoff Switch with Alternator Protection
Wiring instructions here...
http://www.pegasusautoracing.com/pdf...structions.pdf
... you would install this near the OEM battery location, and if you ever decide to race, could easily be setup for external 'off'