How to sell to retailers? As a former Retail Buyer from Target, here's my suggestion. You need to first ready your sales communication and materials to explain how 1) You will drive sales at their stores (best told through sales traction and history in comparable or slightly smaller retailers than the one you are targeting 2) Demonstrate how you will support sales with a marketing plan. Marketing mitigates sales risk and that's a retailer's #1 concern and 3) Prove you will deliver flawless vendor execution. There are many hiccups that happen along the supply chain and it takes an experienced company to flawlessly deliver inventory to the right stores/DC's in the right quantity on the right day. The best way to prove this is by showing that you are currently working with retailers of the same magnitude. So in short, you have to scale up slowly - from small retailers to larger retailers - in order to manage risk, build sales traction, build brand awareness, and manage cash flow.
How to contact them and sell to them? Use Linked In to find retail buyers. I have a short video on how to do this: https://www.youtube.com/watch?v=ZwbfUMRHiq0 Trade shows are also effective; you can get the mailing address of retail buyers in attendance plus meet them face to face if they come to your booth. Also, network within your industry. You'll find your peers are wiling to trade buyer lists with you. But in short, don't bother reaching out to a retail buyer until you have talking points that address those 3 things I mentioned above all.
I'm happy to do a clarity call if you have follow up questions.

This would depend on the product you are selling and the industry you are in. Most consumer products, packaged goods and fashion industries have annual or more frequent trade shows throughout the world. Research your consumer market or the final audience you want your products to reach, then find a trade event and sell to retailers who attend these shows. Having a booth and good presence at these shows also builds your brand. In between events, reach out directly to retailers you want to see your products at. Offer them to sell products on consignments or give them samples to test out demand. The retailer is mostly interested in turning sales as quickly as possible with minimal space requirement, use this knowledge to sell your product. Please let me know if I can be of any further assistance.

You may want to check Doba.com or inventorysource.com. They work with a lot of smaller manufacturers/wholesalers, and provide broad access to e-tailers.