There are nowadays 2 main road brake pad sizes, generally referred to as "Shimano" and "Campag" fitting.
The other bit choice is between "normal" ie Alloy compatible brake pads and "carbon" ie for carbon rim use. The later are not made of carbon but usually some synthetic material which resists heat better as carbon rims do not conduct much heat away and hence braking temperatures are much higher than for alloy.
Finally there is the fixing method. Most quality road shoes are attached by an allen key. In Campag's case by a power-torque bolt which is better but a hassle if you haven't got the key.
Cantilever and V brakes tend to use studded shoes which are unthreaded.
As usual, ask if in doubt.
$13.05 ($13.05 excl VAT) each Sovereign 4Carbon Brake Pads, 1 pair
$10.38 ($10.38 excl VAT) each RavX Road Elite Brake Pads (1 Pair)
$8.20 ($8.20 excl VAT) each Barradine 453C Brake Pads (1 pair)
$8.69 ($8.69 excl VAT) each Barradine 460C Brake Pads (1 pair)
$23.99 ($23.99 excl VAT) each Campag RE-BO500 Brake Pads