I disgaree with only a few comments here but applaud the ones suggesting you go direct to clients to first build up interest. One thing that has remained true in advertising is the need to create a bandwagon effect.
Of course, you will first need to do your homework on what you will deliver to both clients & agencies.
A few quick comments...
Agencies are gated community. Instead of storming the walls (takes time & money) you go in through people who already know the gatekeepers: advertising sales rep firms that work usually on commission-only.
Maybe take a look at partnering with something like The Event Shop (they also offer training for your sponsorship sales staff). But if they do "events" maybe you can create "festivals" with them as I understand they are part of bigger media sales group.
What you should be doing is concentrating on building your platform & platform services. Know your festivals intimately and what specific opportunities they represent by size, geo, audience demo, calendar etc...Slice & dice the info...
For example, it is harder to sell a platform that is both festivals and their attendees than it is to sell two platforms (one for festivals and another for attendees)
And inside these two platforms, target specific clients by creating categories...create segments to get as close as you can...
A technology category for festival organizers is different than a technology category for attendees (one is buying pro and one is buying consumer).
Get involved with hot tech like Big Data... (as advertisers & sponsors want to be of the Next Big Thing not the Last Big Thing...novelty sells)
I can speak to you very specifically about technology sponsors & vendors but there aer others more qualified in FMCG...
One thing to bear in mind: selling advertising and sponsorships is TOUGH. It is NOT "Build it and they will come." No place for the inexperienced unless they have deep wallets to ride out along learning curve.
Maybe a good analogy is The Walking Dead...if you have survived the last 20 years in media & advertising, you must have learned how to be tough, sharp, creative and persuasive. If you are only now stepping foot in this world, find a Rick, Daryl, or Michonne to lead you...
Happy to chat about tech industries...