All of the designers here offer PNG files with the brush sets. So, even if your program can't utilize the .ABR file, if your program can open PNG files, you will be able to use those just like brushes. They can be resized, used as a mask, used as an eraser or cut-out; they can be colored/re-colored, and adjusted via blend modes and opacity. The only limitation that you will have is that you will be unable to "scatter" them, or use them in a dynamic fashion.
The product description will say if the PNG files accompany the brush set (though they almost always do). But if you are unsure before you purchase an item whether or not the PNG files are included, you can always email the designer.