Long before I had ever heard of Mason Pearson ($100+), I began using the Isinis Brush ($17+), which is very similar in appearance but is not a natural bristle. It also comes in a large and a small size. I like the smaller ones better. Not only is it more portable but it seems to work better for me. I bought my first one when I was around 12 at the Carriage Shop at North Park and I have never used anything else since. I have really straight, fine hair that is very thick. Any other brush that I have ever used leaves my hair looking lumpy and separated. The Isinis Brush leaves my hair looking really smooth. And it comes in lots of fun colors and black.
The only bad thing is they are really hard to find. The price can vary a lot too! A few weeks ago, I was at a pharmacy in Highland Park and found them for $40!! My favorite place to buy them is at the Beauty Store & Salon in Austin, Texas. Since I don't live in Austin, my hoarder tendencies come out when I find them and I buy like 3 or 4 of them at a time. They last forever, though so it's a good investment! Another great thing about the Beauty Store & Salon - they have a great punch card program and all of their nail polishes are like 2 for 1 or 3 for 2 and they carry all of the best lines. I love that place!!