You used to be able to get them in any of those workwear shops (it's pot luck which one's have them now they're not that popular anymore). I spoke to a lady in one who said they used to always stock them and could order some in. You can get 'em on online now though around £35.
My dad used to get them from Jay's in Battersea (Lavender Hill). That was about 25 years ago though.
I've still got a pair of square steel toe beige leather work boots I got from there - great shop!
I've still got a pair of square steel toe beige leather work boots I got from there - great shop!