I found my 2D and 3D artists advertising on bunch of sites (reddit r/gamedevclassifieds, tigsource, gamedev, craigslist, facebook Gamedev groups, twitter etc)

Never thought of using social sites like reddit or twitter.

I have used craigslist before, however I find it's mostly app clone or architectural work.

The largest problem I can see with social sites is that, for the client, it will be hard to judge the quality of the artist.

With that said portfolio sites are not much better, as artist will spend weeks on a single portfolio image and only days on a actual product.

I've had some artists approach me after playing a demo I put up on IndieDB.

Community forums for Unity/UE4 are also great places to find artists, and I find that artists who know their way around the game engine you're using would decrease hassle in getting the asset working well and looking good in the engine.

Bear in mind that UE4 (Unity as well, I believe) uses PBR, and to get good looking materials, you need to get acquainted with material nodes. And the top artists in the UE4 forums would also set up materials for you in the editor, included in their price.

It's not just specify-pay-import-done, unfortunately!