Marine Superstore in Port Solent have pretty much everything - just a short hop along the M27.
From the Northam Bridge, your closest would be Shamrock Chandlery (in Shamrock Quay on the Itchen), they're pretty well stocked, then if you head towards Portsmouth along the A27 you'll get Alladins Cave Chandlery on the Hamble just past Burseldon Bridge, closely followed by Andark Diving.
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