Hey since you all the way in Cali might be hard to trade etc...
for escargot shell
Go to a upscale restaurant and more often than not they will give you the shells (might think you are weird) just call ahead etc...
They might smell like garlic butter lol just clean well
I did not notice you were in California. You can buy escargot shells on ebay or amazon. Use an ice pick to poke a hole in them at the top to help get out the air when you put them underwater. You want the largest ones you can find.