Quote: Also.....I've read that the better ones should have two diodes + capacitor
Not so much that better ones have a capacitor but that it is an addition that may or may not be necessary. You can read about galvanic corrosion, the operation of a GI and about the capacitor on Here on the Yandina website
GIs are readily available from the usual yotty electrical suppliers. Pretty sure Merlin do one and anyone else doing much in the way of shore-power stuff.
-------------------- Old Chemists never die, they just fail to react