As above, Mk3 Mondeo calipers are your best option.

The standard Focus has a really weird brake setup where the calipers bolt directly onto the hub, there is no mounting bracket like you would find on most other brake setups. Standard Focus calipers can't be used on anything but the Focus.

ST170's run the more conventional setup where the caliper bolts on to a mounting bracket (which is why you need to change the hubs if you upgrade a normal Focus to run the ST170 setup) but, for some reason, ST170 calipers carry a HUGE price premium over Mk3 Mondeo ones. You should be able to pick up a set of Mk3 Mondeo calipers for next to nothing in a scrappy and they are found on every Mk3 Mondeo so there won't be a shortage of them either!