- Ice Cream & Frozen Yogurt, Northport
- Ice Wholesale, Northport - Dry Ice
- Indian Goods & Products Stores, Northport
- Individual & Family Information & Referral Services, Northport
- Individual Retirement Accounts (I R A), Northport
- Industrial & Commercial Cleaning Services & Companies, Northport
- Industrial & Commercial Real Estate, Northport
- Industrial & Heavy Equipment Repair & Service, Northport
- Industrial & Heavy Equipment Vehicle Rental & Leasing, Northport
- Industrial Contractors, Northport
- Industrial Engineers & Designers, Northport - Architectural Designers
- Industrial Equipment & Machinery, Northport
- Industrial Goods & Products - Hard Goods - Small, Northport
- Industrial Pump Repair & Service, Northport - Centrifugal, Positive Displacement, Submersible, Diaphragm, Hydraulic, Water, Vacuum Pumps, etc
- Industry, Northport
- Infant Clothing & Accessories, Northport
- Information Retrieval Services, Northport
- Inspection & Testing Services, Northport
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- Insulation Contractors, Northport - Blown-In, Spray Foam, Batt, Fiberglass, Attic Insulation Installation & Service
- Insurance Carriers, Northport
- Insurance Claim Processing Services, Northport
- Insurance Farm, Northport
- Insurance General Liability, Northport
- Insurance Motorcycles, Northport
- Insurance Renters, Northport
- Insurance Services, Northport
- Interior Designers, Northport
- Interior Designers & Decorators, Northport
- Internal Medicine Doctors, Northport - Internists Physicians & Surgeons
- Internet Marketing & Advertising Services, Northport
- Internet Products - Hardware & Software Providers, Northport
- Internet Services, Northport
- Investment Management Services, Northport
- Investment Services & Advisors, Northport
- Italian Restaurants, Northport
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