i got a pack of 10 or so for a dollar.
they start falling apart after one round of resin (sponge bits everywhere) so use a new one. you can afford it. and it's not fun having concrete hard sponge bits on your armor.
I use cheap brushes myself and to prolong it just a bit longer, I soaked the brush with acetone after use. Mind you, you'll have to soak it and clean it off with a rag. That means straight after use, keep 'washing' it in a small container and drying/cleaning' it off with a rag. It'll keep the bristles soft enough for reuse again.
Point to note, DO NOT leave it in the acetone container as it will harden up. Trust me, I made that mistake and had a nice brush sculpture after.
I use a cheap bristol brush for coated the first layer or two, works great. Then, I use fiberglass jelly and apply by hand using latex gloves, this way you can sculpt the piece alil more will less waste and sanding.