Banana plugs are very common in audio equipment and some top-of-the range
Hi-Fi, so I would try Maplins, RS Components and specialist audio shops
rather than any of the scuba shops.

Bananas are most commonly used on Hi-Fi speakers as the terminal posts on
Hi-Fi speakers normally accept either banana's or bare wires.  (central hole
for banana plug with external screw-down thumbscrew for bare wires.