Purdy tools are available at independent paint stores, hardware outlets and home centers. Most synthetic brushes work well with both latex and solvent-based paints, but always check the manufacturer's recommendations on the brush. High quality or more expensive brushes have distinct advantages over the cheaper ones. First of all, a high quality brush will finish the job more quickly. Also, a top-quality brush will have a tapered end, which means there are shorter bristles on the outside and longer bristles in the center.
Tapered bristles give the painter more control over where and how much paint goes onto the surface.
Generally you need to buy two brushes. Run your hand over the cover to see if it sheds any lint. If it doesn't it's probably a good cover. Check both ends of the cover to see if there is any fabric hanging over either end. Look for the seam in the roller cover.