Hi Darryl,
Keys come in all shapes and sizes. The best plan is to measure up the slots in the shafts and try local hardware or autoparts shops to see if they have something.
There are broadly two types: the woodruff key ("half moon" - think of a segment cut off the edge of a coin) or the parallel key (usually almost square in section, usually with rounded-off ends). If you can't get the size you need, get a larger one and file it down to suit.
Keys are often quite soft steel, which I like as the taper usually carries most of the load and I'm happy for the key to shear if the taper joint fails. You can make your own keys from mild steel.
The most common mistake is to use a key that is too high. Make sure it doesn't prevent the sprocket from seating on the taper.
Cheers
Leon
