Yep, Rixter & Indifference are right. If I look back over the houses with flats that we have purchased for clients (and ourselves for that matter), the vast majority never hit re.com or domain.
We have been doing it for so many years now that we frequently get calls from selling agents before they get marketed. We also get the first call if a sale on one of these falls over for some reason. We promise them a 4 hour response time to inspect the property and an answer Yes or No either 'on the spot' or first thing next morning after an overnight discussion.
It works for us (and the agent too).................and we are not afriad to say No either. We probably say No, 2 times out of 3 for one reason or another. After a while the agents get a feel for what you do and don't want and then don't call you with unsuitable properties. It can take a bit of training until you get them just right.
So, you've been given good advice - it is all about building the relationships with the people that are invited to list properties every day - the selling agents.