Hi Andrea: there are a few "matchmaker" companies that I am aware of that help start-ups find sponsors/marketers and vice versa. There is http://www.startupsforbrands.com
from my friend and co-author @jaffejuice
. And in the UK thereis http://t2blimited.com
from my friend Ken Valledy.
These companies aren't necessarily finding you sponsors, but can help in finding marketing partners that, in turn, can help with development/expansion/reach and visibility of your start-up and as a result will help in attracting more users, funding or other growth related opportunities.
There are also a growing number of marketers and media companies that have set up incubator/funding infrastructure:
It is INCREDIBLY hard to find sponsorship/partnership opportunities on your own. Many companies are inundated with requests, and don't have the time or infrastructure to assess or evaluate the opportunities that are coming their way. The company's mentioned above with the start-up ventures are really the exception to the rule. In most cases you may never get an answer to your first introductory pitch email or call.
Your own network has some value in this respect. If you have connections that can open a door to a meeting with a potential partner/sponsor you stand a slightly better chance. But remember: for them, you are one of many, many things to deal with, to understand, to evaluate and you are very low on the priority list. I recommend to read the blog posts by Ken Valledy at T2B as they explain the process from the other side which is going to help you with your expectations and understanding.
That is why companies like Evolution or T2B exist - they make it easier for start-ups and marketers to find each other.
I hope this helps.