I have exceptionally narrow flat feet and have only ever been able to find one pair of flip flops that fitted me - a pair of Oxbow ones with an additional adjustable strap going across the foot. Every other pair I have tried just fall off my feet! My darling much loved Oxbows are now 9 years old and have seen better days. Needless to say they no longer make them so I am on the hunt for very narrow or adjustable flip flops and thought I'd ask for some help here! 
I've just measured my feet; at the widest point they measure 13cm if the tape is touching the floor either side. Last time they were measured at clarks (15 yrs ago) as a 15 year old they were a 'b' width fitting. I'm a size 6 but usually wear a 5 in sandals as they come up a little narrower. I specifically want flip flops - I don't have as much problem with sandals as Mephisto fit me (in a 5) and I had some others made for me last year.
I have seen Havaianas do a 'slim' flip flop but how slim is slim? We don't have any good shoe shops around here for me to browse/try on so am looking to order online.
Any suggestions would be very great fully received. I'm not too fussed on colour/design I just really need a new pair that are going to fit me. Tia. 