Here's Dick Blick's brush index page for an overview:
For a stiff synthetic bristle I like the Cryla brushes from Daler-Rowney, don't know of their availability online but a quick search in Google will sort you out. I'm not aware of any equivalent made by anyone else, sorry.
In softer synthetics - rounds, flats, lange du chat/filbert, angled shaders and liners - I use golden taklons mostly, made by a number of different people including D-R, W&N and another English brand you won't be able to find in the US. There are equivalents in many American ranges.
I use Kolinsky rounds for painting detail, there are lots of good brands. I would suggest trying Da Vinci, Raphäel or Escoda over W&N's Series 7 which are too expensive and not as consistent as they used to be. Dick Blick also have a proprietary Kolinsky but I haven't had a change to test mine yet.
For more opinions I suggest doing a search here in the Acrylics forum and you'll find a number of threads on brush preferences, there was one a few months ago if I recall correctly.