You're in a very common position for a Startup. First, I would say that buying and selling is a very human-oriented trade requiring a sincere, human-touch approach often times, and therefore the industry you're in and the manufacturers you're looking to source products from will require different tactics than those of the startup down the street.
Without knowing your industry/manufacturers, my suggestion is to 1) be beyond passionate about your forthcoming business and the reasons for wanting to sell their products and 2) be open and honest about exactly where you are in the planning process, and ask them for all the information on their wholesale/distributor operations as possible, regardless of whether you are in a position to place an actual order yet per their policies.
These two tactics put you on their radar, help make a case for why they should do business with you, and sets the stage for further communications once your site is ready.
Like a good business plan, including details about yourself and your principals (the 'human touch') can be a great selling point. And of course a logo and any branding you already have in place short of a complete website will go a long ways.