I have yet to discover a good, affordable brush. I can, however, tell you what brand NOT to buy.
Don't buy Blick's acrylic brushes because they are terrible. As soon as you use them they completely lose shape and the hairs get all frayed and look as if they've been used for years.
The quality is about the same as a set of student brushes I bought for a children's party. But the Blick brand was much more expensive. In fact one of my favorite brushes to use is a cheap flat brush that I buy at Michael's. I may just go back and buy some more. It was an expensive lesson to learn and I hope this little tip helps someone out there save some money.