Wait, wait, wait, WAIT! I'm the breeder here! XD
All of the suggestions above are correct, but let me try and get you a more complete answer.
1.) Theres always the option of ditto. By suggestion here is to teach your male Cloyster some moves that for Shellder to learn as egg moves.
2.) Breed Cloyster and Ditto together to get a female Shellder. Then breed it with another pokemon in the Water 3 Egg Group, to get a special move or two.
Yeah, the only way basically to get a Shellder is to breed it with Ditto, but you could go and do a lot more then that. My advise is to teach Cloyster Shellder's egg moves, and breed it, to get a kick-butt Shellder.
So yeah... Hoped this helped!