The brake kits are usually how you replace the brakes on your trailer.

I'm sure you can purchase the shoes only, But why? The difference in cost is so low there is no reason to not buy the kit.  If the shoes are worn, You probably also need to replace the magnets and springs also, which the kit includes.  The kit makes the brake work a snap.  Pull the drum off. pull the kit off and slide the new kit over the exposed lugs. Service the drum and bearings and you are done.

But how much do the magnets cost. Just a quick look on Trailer Supply shows that the magnets alone are $19.35

http://www.easternmarine.com/dexter-12-dxq-electric-brake-magnet-kit-white-wire-k71-105-00

Shoes along are $32.85

http://www.easternmarine.com/dexter-12-x-2-dxq-electric-brake-shoe-set-52k-k71-048-00

So you are paying $52.20 per hub.

Where you can buy the whole assembly ( put together and ready to bolt on) for $34.99

http://www.easternmarine.com/12-x-2-electric-brake-assembly-left-hand-52k-k23-105-00

I don't purchase from Eastern Marine, They were just the first site that pop'd up on a google search. But their prices are reflective of what I'd pay my local parts house.