I got one from Primark for £3 this weekend. Not sure if you need any but I got some swimming shorts for J, reduced from £7 for to pairs to £2, they are lovely as well.
I also managed to get him some shoes this weekend so we are all done for our hols. I went a bit mad and got him 3 pairs of boat type shoes in 3 different colours and a pair of sandals from Next. I did quite well this weekend as I also managed to get E 5 bikinis from Primark, on of them sounds like the one you mentioned that you got for your SD, like a hawaii type one so thatnks for the recs.