Sonia Kashuk brush sets from Target, they are affordable and always come out with new sets! They also have a great starter eye brush kit for $12.
Real Techniques has great brush sets for around $20 each. Available at Ulta, Target, Walmart, Walgreens or online!
ELF makes a brush set of all their $3 studio brushes for $30, not the highest quality brushes but great for the price! Available online only at eyeslipsface.com
Sedona Lace makes great quality brush sets, professional quality. More on the pricey side but a GREAT investment. Can buy them online here.
Sigma makes fab quality brush sets, professional quality. Also more on the pricey side but totally worth the investment if makeup is something you take seriously and do often. They make many different sets ranging from eyes to face to a mixture of both. Can buy them online here.