The Frenden brushes are fabulous, I will definitely back this up! They're worth every bit paid for them.
You can also find a variety of brushes on the official site. There are so many here, and you're sure to find at least a few that will really work beautifully for exactly the piece that you have in mind. I've used so very many of these, and they're wonderful.
Last but not least, there are plenty of free brushes out there. The advice has been good so far! There are also brushes you can sometimes find on tumblr or other social media. I personally offer a few free brushes too! Most of mine are created for specific purposes, but these specifics have proven really useful for me and, I've heard and seen, useful for other artists too! If you enjoy my brushes, I hope you'll let me know. 
Good luck on your search!