Hiphop - usually by word of mouth, people telling me about albums/groups, etc.
Punk/metal/ska - I pretty much don't like any of the new bands coming out these days so i'll just occasionally buy a cd if one of my old favorites does a new album. Occasionally I'll go by Headline Records in LA and see whats up. Amoeba never has anything good though.
DnB/Reggae - I waste hours and hours of my life scouring through dead links and seedless torrents to find obscure sets, dubplates, and rare gems. Soulseek used to be awesome for this because you could actually hang out in chat rooms and talk to people about what awesome sets and albums they had, then just dl directly from that person, but no one really uses soulseek anymore.