Choosing a specific supply business to invest in depends on various factors like market trends, your expertise, and personal preferences. Some popular options could be:

1. E-commerce Fulfillment Services: With the growth of online shopping, providing warehousing and order fulfillment services to e-commerce businesses could be profitable.

2. Wholesale Distribution: Distributing products to retailers could be a stable option. Look for products with consistent demand.

3. Medical Supplies: Given the ongoing importance of healthcare, investing in medical supply distribution could be a resilient choice.

4. Food Supply Chain: Food distribution, especially organic or specialty foods, can be lucrative due to consumer demand for quality products.

5. Construction Supplies: Providing materials like lumber, hardware, or building equipment for construction projects can yield good returns.

6. Renewable Energy Equipment: As the world shifts towards cleaner energy, investing in solar panels, wind turbines, or energy-efficient products might be wise.

7. Office Supplies: Despite digitalization, there's still a demand for office essentials like paper, pens, and stationery.

8. Automotive Parts: Operating in the automotive aftermarket, selling parts and accessories, can be a steady business.