For my own work I prefer to use Robert Simmons Signet Brushes.  You can purchase these at almost any art supply store.  Especially useful for abstract painting are the following sizes:
#12 Bright
#14 Flat
#6 Round
#4 Round
#10 Filbert

If you can't find Robert Simmons Signet, Grumbacher also makes good bristle brushes.

Also useful to have in your repetoire of brushes are several larger brushes that you can buy at hardware stores or Home Depot. Just be sure to get bristle brushes, not nylon. In addition you can add some sponge brushes and a scraper or spatula.

For glazing or blending it is useful to have a badger blender or soft synthetic sable or real sable brush.