For a Raleigh you need 3/8 inch cotterpins. Conveniently this is the most commin size so should be easy to find.
French and some other continental made biked had a slightly smaller 9mm pin which won't work with youd bike.
The secret in installing these is to know that the nut is only there to hold them in place once they havd been hammered or pressed into place. Thd threads will not be strong enough to pull them into place, you'll either strip the thread or else have loose pins ten miles down the road.
You may need to file them slightly on the tapered part so that they go in enough to get good purchase on the spindle. Also, if you study it, there is a right a wrong way to put the pins in. It will make no functional different but the nut and threads can catch the leg of your trousers.