I use those sizes Darth mentioned, I usually go to my local art/craft store as they sometimes have them on sale and they're artist quality. I got all the ones I have now in a box of stuff I bought but I'm due for new ones nwo that I'm doing sculpting. I also go to the dollar store and get their 1.00 packs of the generic plastic handled brushes you use as a kid to water color in school. They're great for doing large areas, just sometimes you have bristles come out but a good prewash could probably fix that, and when you get a dozen for 1.00 you can just throw them away instead of worrying. We use them for putting varnish on knife handles too.
Personally I use Games Workshop brushes, easy to get a hold of but quite expensive, I'd suggest a standard brush, fine detail brush, fine dry brush and large dry brush. I use a cheaper brand called Javis (size 4 I think)for undercoats and large areas.