Well, from personal experience, I'd say it's this:
Go to the Duke in Lumbridge Castle, talk to him and he'll give you an anti-dragon shield (300gp each now, I think). Keep talking to him and getting shields, there's a bank on the next floor up of the castle. =3
You used to have to drop the shields and pick them up, then note them at the bank and withdraw, but now they've updated it so that you can keep getting more shields from him if you have some already.