How do I go about introducing an Idea for a poster/T-shirt to company that might be interested in?

Answer 1

You don't have to. CafePress.com lets you make your own items using your own designs without having to shell out any money upfront. Check it out:
http://www.cafepress.com/
Click on "Sell Products Featuring your own unique designs."

Of course, you'd have to promote the items yourself.