Piratejohn; first off welcome aboard Shipwreck Forum!
I am sure that you will be able to find a magnetometer, either directly or indirectly. If cost is a factor (it always is for me), then you may be in the market for an older proton magnetometer with analog data outputs (usually paper recorder), and not as sensitive as the newer units available today.
The newer magnetometers today are using cesium vapor or hydrogen, and all utilize digital technology.
If you are technically skilled, there are even links to sites with plans to build your own magnetometer at the Geophysics link on this site.
Good luck and happy hunting
Q