If lobster stock is unavailable, clam juice or shrimp stock are excellent substitutes. Lobster Stock Recipe
A concentrated paste (like "Better Than Bouillon") that is cost-effective and lasts longer in the fridge.
Many seafood markets sell leftover lobster bodies and shells specifically for making stock at home.
Freshly made or opened stock lasts 3 to 4 days in the fridge. You can freeze it in ice cube trays for up to 6 months for easy portioning in future recipes.
Often found in the specialty or seafood aisle. Quality varies, so checking reviews of common brands is recommended.