Firstly, it's worth noting that several podcasters I've spoken to find that genre-specific podcasts get more downloads than those that play many genres. In a sense, it's easier to find tracks that fit together style-wise. Artists' webpages etc will have links, 'Likes', etc to other similar artists. Their readers will like the genre of music they make, so there is a built in list of people to advertise to, as well as whose musical tastes can be mined for ideas. Here are some sources we use to find new music.
SoundCloud
SoundCloud is a treasure trove of useable tunes. It's not really designed for easy browsing, though, so you may need to search using tags or keywords that relate to the genres you want to play. If you want endless, homogenous, unremarkable trance, you're in the right place. For more interesting genres, you'll need to dig around a bit more but there are real gems to be found. One slightly annoying thing, though (I feel slightly guilty saying this because we do it ourselves) is that tunes aren't always downloadable. Sometimes they're commercial releases that aren't out yet and are on SC for promotional purposes. Sometimes I message them and ask for a 320; I've been able to play several tunes ahead of release date which is great.
SoundCloud Groups
SoundCloud, like many other sites, has groups which are meant to be ways people with similar tastes can share items they've uploaded or found. In theory, this should be a good way to find likely tunes. I created a group, Your Tracks for our Podcast. Despite a clear and concise description of the kind of tunes we want, however, morons keep posting trance, techno and house tunes. I instantly block these people - they are either illiterate cretins or deliberately annoying gits; either way, I don't want to deal with them. Not because they're into those genres but because they're bloody annoying. If you look, you'll see how blunt I've had to be with people posting tracks. Anyway, you may be luckier - I have acquired several top tunes for the podcast through this group. I also have a group not specifically aimed at finding podcast tunes and, paradoxically perhaps, it's been a better source: Transnational Beats
MixCloud
Listen to mixes or podcasts playing similar genres to you. Find tracks you like then track down the artists or labels and ask for tracks. Simples!
Commercial outlets
Commercial outlets can be useful. I buy tunes from Addictech. I can get decent WAV quality there. I used to email labels and artists asking permission to play their tunes. Once a label had said yes, I presumed they were always happy so I didn't message them again - I just use their tracks. They've had a sale out of me after all. No-one has ever complained - quite the opposite, in fact. People seem to be happy we're promoting their music (as they should be!) so these days I don't ask permission. I do, however, always make sure commercial tracks have a cross-fade with another track, beginning and end. This might help my defence if ever one of the labels got stoopid about it.
Musicians
If you do a nice job of promoting artists in your podcast, they are very likely to send you new material. Hey, free music! It's always nice to be able to play tracks that aren't quite released yet. Have business cards and hand them to producers and artists at events. Get to know musicians. Offer a short interview too and they'll be throwing tracks at you.
PR companies
We have a couple of music PR companies that now send us items for review in our blog or podcast play. I think they mostly find you, based on the buzz you're creating.
Last FM
Not the easiest site to use but it does recommend tracks based on your listening habits - 'scrobbled' from mobile devices or your home computer, not just plays on Last FM itself. There are many free downloadable tracks on Last FM.
Jamendo
Jamendo has over 340,000 tracks that are available under a Creative Commons license. Details HERE.
