I like springy synthetic brushes but im pretty hard on them so i stick with the cheap blue handle ones. If you are looking for a nice brush this link will get you a nice #2 with a full body for holding paint. Just treat it nice and avoid painting with the tip.

The best detail brushes for miniatures are Kolinsky Sable hair paint brushes

I haven't used these, but this looks like a good deal with good reviews if you are looking for the cheapest option.

Winsor and Newton is a more well known option, but much more expensive.

If you aren't locked in to Amazon only, Blick Art Supplies is a good option for brushes.

Make sure you practice proper brush care.