As your Fullfillment By Amazon (FBA) account grows, you'll want to find cheaper sources for larger quantities of products, and wholesale is the most convenient option. Shifting to wholesale (as opposed to raiding the discount shelves at a big box store for products to resell) takes some adjustment, but it often leads to more successful sales and larger profits. That is, if you find the right wholesalers and the right products.
Rarely Appearing Doesn't Mean Rare and Desired
First, keep in mind that a product that you find that doesn't appear to have many sellers on Amazon might not be a rare product -- it could be that there have been problems with the suppliers resulting in poor product quality. In other words, say you see a product that appears to sell decently but that only one or two sellers seem to be selling. That could be an opportunity for you, but be careful because it could also be that there are few suppliers willing to work with Amazon FBA or who are able to provide quality goods. Check reviews, especially the one- and two star reviews. Look through the related-items lists that come up, too, because you may find similar products that tanked.
You May Need to See the Physical Product
Unless you land a deal with a well-known wholesaler for known brands, you may need to see the product, first. You could try ordering a version off Amazon from another seller who uses that manufacturer and see what shows up; if the product is good, then you know that it's an option for you to sell. If you don't want to give your competition money (which is understandable), see if you can order a smaller batch from the wholesaler to be sent to you -- and make sure you know about return policies.
As successful as your FBA sales may have been so far, wholesale is a different animal and can often be awkward at first. Start slowly. Work with one wholesaler for one or two products and see how those go. If the wholesaler is good and people seem to like those products, then you can look at what else the wholesaler has while also seeking out additional companies.
Remember that your storage fees will be affected, and you may want to look into where your products go in terms of distribution center location to help contain those fees (there are ways to hold products so that you have larger amounts going to the same place). Wholesale products can be lucrative additions to your FBA account if you find the right wholesale sourcing for Amazon FBA..