No. 1 Son is a BMX fiend (day out at Corby indoor park today to get more scars that chicks might dig, apparently). He has a new bike; front wheel has a 10mm axle, rear is 14mm, this now appears to be standard (or as standard as BMX bikes ever get).
Pegs are either 10mm or 14mm, and can be bought singly to mix & match. Alternatively, spacers can make a 14mm one fit a 10mm axle, but they can be a pain to fit. Pegs from BMX shops tend to cost £"howmuch?"
Also, all his pegs are bolt on, so tpi matters not. Just held on with axle nut.
Try local Halfords for screw-on pegs from cheaper 'BMX' bikes (i.e. not ones you would dare to jump for fear of total collapse)?