I have been going after drops for a long time, but on a very small scale compared to some of the folks we talk about here. I went after names several years ago using just a simple Whois monitoring script, but times have changed.
I've had the most success with Snap and NW, and to a lesser extent, NicGenie. I generally hedge my bets by using a shotgun approach when there is a name dropping that I need. I've occasionally used 'private' sources for a particular dropping name that I already have a buyer for, but in today's climate, this is usually not cost effective for me. I'm sorry that I cannot share more about some of my other acquisition resources, but grabby's gotta put food on the table, too.
I am nowhere near a big enough player to currently compete with those who have their own registrar deals. Especially lately, it seems that Buydomains and Ultsearch have been pretty dominant. It's a futile gesture I know, but I've moved the couple of names I had at a few of the registrars who've sold out to BuyDomains (Domainzoo and 1eNameCo are the obvious ones, but I'm sure there are more) to other rar's that I like better anyway.