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.
Importing Custom Invoice Templates into QuickBooks Online
Thanks for the endorsement @Anonymous
@snance. I agree that Google is a great resource for ideas. I've included some templates I created with artwork and images I found on Google. I used MS Word to remove some of the fields in the table in the default template as well as used watermark in MS Word with some of the images. Almost anything you can do with MS Word can be imported into a custom template.
Something you definitely want to keep in mind when doing custom templates is if the majority of your invoices are going to be emailed to your customers or printed and sent hard copy. If email, then go crazy with pictures, color, and anything your mind can come up with. If you're printing, you can still go crazy just keep in mind things like how much ink or toner you're going to go through. For example, I wouldn't recommend doing a template with a solid color background because when you print it you're going to blow through a lot of ink/toner.
Good luck. Hope to see some of what you come up with