talkmagic.co.uk

Anyone know of a source of A4 sized metal, paper thin and magnetic?

I do not have exactly what you are looking for but I do have an alternative that, IMO, is better* than what you are after if you are willing to go the dry mount route.

http://www.lessemf.com/mag-shld.html#278

Metglas shielding ribbons. You'll want 1" wide and .00065" thick.

*ETA- that may not be a fair way to put it. I have not made a shim the way that you are looking to do. I have made it the way that I am suggesting, a dozen or so times, with great and undetectable results.

The dry mount gaffing I do simply follows the Advanced section of The Gaff Factory by Craig Matsuoka. It can be found on Lybrary.com for $20 USD.

Try googling magnetic foil, I've never used it but have often wondered about it.