At this stage good Graphic Designers, Copy Writers and Marketing Managers are critical to the success of the project.
In this scenario, agencies are ideal.
I have a background but am by not means a professional in the field.
In very tight projects I do end up having to help build mock-ups and here are the resources I use.
Design Templates
http://www.templates.com/ - collection of free and paid templates.
Icons
http://www.iconfinder.com/ - search engine linking to many free icons.
Fonts
This has its own section.