Last year, in preparation for an away weekend, I bought a little set (approx $9.99-$12.99) of EcoTools brushes to take with me. They were amazingly soft, considerably cheaper than others, and have become my favorite travel set.
Here's the jaw-dropper: they're made from bamboo fiber. So they're not synthetic, and they're not animal-based. For those with skin sensitivities or moral aspirations, these can fit both bills.
Today I treated myself to another little set, but of kabuki brushes (approx $12.99-$14.99). I'm looking forward to the short handles for added control, and the soft, specially-shaped brush heads for each particular application.
Best of all, these brushes are available in most drugstores (like CVS) and beauty stores (like Ulta).