It's better to catch them yourself. That's what these bait shops do. I live in Everett. My first Oscar loved sand shrimp. They are easy to dig up.
Edit: I know this because one day I was walking along the beach and saw about ten people digging and loading buckets with sand shrimp and I was like, "Wow. Gonna have a barbeque, huh?" They were like, "No, we all own bait shops and know the best times to collect these guys."