I think the artist never intended to give you the original oil drop image in the first place because he does say "now add a photograph" not "now add my photograph".
A few resources I know of include:
- Deviantart, do a search for any brush type + the word brush (i.e. oil brush) and you will get lots of free downloadable brush types. This also is a great resource for oil images as well to get you started.
Oil Brush Search on Deviantart
- Brusheezy has some free brushes and patterns you can try out such as this one:
Brusheezy Oil Brush
- General list of brushes from a simple Google Search of Oil Brushes. Just use google image search to find a similar image for the oil drop.
Google Search for Oil Brushes
Google Search for Oil Images
Finding exact images that you want will be tough since that's for you to decide. Please remember that all of this research can be done by yourself if you just search hard enough, these are simply suggestions for ways to start looking.