One of our moderators, @LisaNullar, wrote an article about how to Import Custom Templates using Word. This gives you an incredible amount of flexibility for how you want to lay out and texture your templates.
Google is a rich resource for getting ideas, but you have a great opportunity to spread your wings a bit. Use what's out there as inspiration. Go with your gut and make iterations on what you see to make your proposal templates your own. You know your clients and business best, which means you're going to have the best insight for how to design them.
@MarshallA has shared some pretty spectacular templates in the past, maybe he can share a few here.