Hi Rob,
Great question! I find my new pickups in order of frequency:
1) Networking with many other like-minded collectors
It helps to have a large network of close collector and dealer friends because you "cast a larger net" in the hobby. You can't possibly see every auction and be all-knowing without other friends in the hobby looking out for you. Many of these friendships were forged long before the internet. Without my network of hobby friends, I wouldn't find half of the stuff that I do. Seriously!
2) eBay favorite searches
3) Large auctions
4) SCD (Yes, I actually still subscribe to it and it sometimes pays a dividend. Old habits die hard, even if I may be the last remaining subscriber)
4) Smaller auctions (Infrequently)
I hope this helps.