What would it take to start a modest vertical ad network?
Anyone who tells you that you first need to gain access to sites "so that you have something to sell" is simply wrong. You will not succeed as a one-man shop if you attempt to go that direction without deep capitalization. Find the advertiser first. Get a commitment. Then go find traffic you can sell to your advertiser at a price that leaves a profit margin for yourself after you've paid the publisher. Other advertisers and other publishers won't be hard to find when you have a happy first advertiser and a core of happy publishers. Find the advertiser first. Good luck to you.