How do I approach a potential supplier/manufacturer to supply their products for my new business start up?

I wish to list there products for sale on my website but my company (and website) has not started up yet!

So here's the problem! The company’s primary service offering is not retail or sales... but is to provide technical services, such as maintenance and repairs for example! and so the company will only take up the retail line of business if I can negotiate and order from a sufficient amount of suppliers/manufacturers.

But with no up and running business profile to show them, how do I get them to take me seriously??.........

What would that initial letter say?.. Include?.. To sow the seed, so the prospect of a long term working relationship is a reality from the outset and my letter is not just put straight into the shredder!! ……… So back to my original question:

How do I approach a potential supplier/manufacturer to supply their products for my new (“proposed”) business start up?

Hope you can help ? :-)