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.