If You Are Just Starting Out

1. Garage sales and flea markets: If you're starting out small and want to experiment with eBay, garage sales are a good place to start. Do some research into categories that interest you, and then start scouring sales for good prices.
2. eBay: eBay itself can be a source for products. Look for wholesale lots that can be broken down for individual sale.
3. Drop Shipper Directories Drop shippers are wholesalers that will ship direct to your customers so that you don't have to invest in or store inventory. Directories of drop shippers are for sale online, but make sure you're buying a reputable one.
4. Local Businesses: You may find businesses in your backyard that offer just the right products, but are not yet into e-commerce. Offer to sell their product online in exchange for a percentage of any profits you make.
   
   
5. Crafters: Local crafters are a good source of unique products, and may be willing to reduce their prices for you if you purchase in quantity. Either buy the item outright, or set up a consignment arrangement with them.

For The More Ambitious

6. Trade Shows: Trade shows are my favorite way to source products. Lots of merchants gather in one place to look for resellers. To find trade shows in a particular industry, contact trade associations and industry publications.
7. Permanent Marts: Many industries have a location that houses permanent wholesale showrooms. For example, there is a "Gift Building" in New York, and a Furniture Mart in High Point, North Carolina. Once you have established yourself as a retailer, you can visit the marts and have face-to-face meetings with product suppliers.
8. Wholesaler Directories: Your local library will probably have directories of manufacturers, wholesalers, and/or distributors. Most directories are organized by SIC code so that you can zero in on the product categories that interested you.
9. Importers/Exporters: If you are thinking big, you might want to consider contacting companies that import goods from overseas. If you're thinking even bigger, it's possible to source directly from overseas. This requires a lot of expertise, but many companies do it successfully.
10. Foreign trade offices: Most countries maintain trade offices in major US cities to help their nationals expand trade with the US. They may be able to let you know about companies interested in exporting to the US that you can contact.

SOURCE http://onlinebusiness.about.com/od/shoppingcarts/a/productsourcing.htm