Where can I find a network for small businesses looking to help each other promote their companies/brands?

You can look in your area for an organization devoted to promoting small and local businesses. Here is an example.

Online, try meetup.com or linkedin.com

Also check out associations that do the same type of work as you do. If your services are more marketing-geared, join a marketing association. They are a great referral to tonnes of business.

Look into your local commerce.

LeTips

Here they call it the Chamber of Commerce. All the businesses that want exposure band together and put out pamphlets that are available to the general public and visitors.

this is a bit different, but merchant circle is a good way for small businesses to advertise locally.

http://www.merchantcircle.com