Okay, this is a tricky one.
I can only advice you to contact the artist beforehand and ask for permission.
That you give credit is a REALLY good thing and you will see that most people are ok with you using their stuff.
Downloading brushes is fine, look around deviantart, many digital artists offer free brush-sets!
If you use a drawing were you do not know who did it...just don't use it.
It's all I can say about it.
Google actually helps you with finding the owners.
www.google.de/imghp?hl=de&… Just upload the picture and google tells you were you can find the original artist. (Most of the time)
Hope this helps you a little.