These are the ones I used on my fog lights. I could only source them from the US so they are quite expensive. However, there is a version sold over here which I saw at Billing. Can't remmember what they're called though.
try ringing around companies who supply winches, these do them. devon4x4, 1stfour4x4 prop do them. also try the OEC. you should come up with something.
If you don't want to spend a lot and know you're not going to want to take the rack off (without having to cut/drill the fixings) these would probably do:
what i did with mine was pop rivet deck eyes from a chandlers,and made up s/s wire with an eye at both ends then padlocked it to the roof rack also riveted the clamps that hold it to the gutter.
what the thieving basturds do is lift the whole rack and tent onto another landy thats parked behind it then fuc off