You haven't described the service you're offering, so it's difficult to know for sure where to send you. If your service obviates the need for a marketing team I wouldn't recommend it (even if it's marketing based).
For a walmart/bestbuy I'd try to win a single store first if that's possible. Or target a smaller competitor and use that as leverage. If you're trying to sell to an entire F500 at once, I'd probably say you need to change scope. Perhaps there is some way to piggyback without their permission? Making it so they can't ignore you.
Large retail companies have "buyers" or "merchants". You want to contact them. Product samples, schedule calls, and be prepared for face-to-face meetings which should be Goal #1 (that's where you will close the deal). If you don't already know - Ask them how they view their average Consumer, then prove to them why those Consumers will love your product.
Make the risk as low as possible for them. Fund the inventory, fund the advertising, etc. - these are things they will try and negotiate. In the end, business is business. If they think their Consumer may want your product and the risk of trying it out is low, they may just move forward.
Bonus:
The chain typically goes like "assistant", "associate", "buyer", "manager", "vp" jump in the middle (or lower if you need to) and work your way up.
EDIT: Also there are Vendor Rep companies that have these relationships. This may be something you want to consider.
For a walmart/bestbuy I'd try to win a single store first if that's possible. Or target a smaller competitor and use that as leverage. If you're trying to sell to an entire F500 at once, I'd probably say you need to change scope. Perhaps there is some way to piggyback without their permission? Making it so they can't ignore you.