Can get them on amazon mate. Get a few of them if i was you they run out quick and people seem to get sticky hands around them. Also try Ryman if you have one near you. Sometimes referred to as paint pens i think.
The silver sharpie looks white when you write on dark coloured clothes, I know it's ok because my lad only passed out of Raleigh in October.
They don't care what you use as long as it marks your black socks etc, but as I said the other ones wash out so you need to keep remarking but not the sharpie.
Good luck Flinty.