Great question! I love questions like this ones.
First off forget about Silicon Valley. If you are not here with that sort of relationship you will spend a lot of resources before you make something sustainable here.
Second, you mention you've had some success - ok, why aren't you simply making that effort that has worked more efficient and thus grow that effort scale to in return scale up your success rate?
Emails are for long term nurturing not for quick generation. What I would suggest is first never show up empty handed, when you do make contact with one agency, know what they do and who for, offer directly related solutions for 1 client only. If you do a good job there they might build the relationship on their end.
With that said, don't pitch for long term pitch for an opportunity to save money or time or etc once.
Sorry but I gotta go, short reply.
If you have any further questions let me know :)