You need bulkheads that fit the size hole it is. Usually 3/4 or 1 inch. After you install those then you can put a ring of silicon around the edge of the bulkhead if it won't hold water....
Yes, my lfs has all types of bulkheads in stock. They charge triple what Lowes or Home Depot charges. I second the suggestion to use silicone to eliminate water seepage....
In addition to the silicon, don't forget to use plumber's wrap/tape on the joints (thin white tape you wrap around the treads of the bulkhead screws) -- makes a tighter connection
