They do allow for leg movement - There's quite bit of leg movement involved in horse riding. The simple act of mounting involves a lot of leg movement, those legs have to be free!
I got mine from a local tack / saddlery shop which sells jodhpurs, gloves, riding hats etc. When I rang to see if they stocked them it caused much mirth, as the woman on the other end thought it was a mate of hers just winding her up, this from a woman who knew full well what they were! Mine had a suede seat which was very comfortable sadly I can't remember the make. They really do stop you slipping, how much sliding do you require when you play?.