Called for in Grandma Zaino's Linguine with Clam Sauce. I'm familiar only with bottled clam juice. Can you make or buy fresh clam juice?
Anywhere that sells fresh clams. I always strain mine through a coffee filter (to catch the fine grit) before using.
Please keep it in mind that it is highly perishable (as compared to the stuff in the bottle). Don't let it get warm and use it as quickly as possible after purchase.