They are readily available at any reputable wheel and tire shop. Just make sure the hub bore of the wheels that you are buying is at least 63.3mm or the wheel won't fit over the hub.
If the hub bore is larger than 63.3mm, the appropriate size ring will make the wheel fit perfectly on the hub, so the lug tightening will not have any negative effects.
Don't forget about offset for the wheels. A typical Focus can handle anywhere from 40mm to 52.5mm. Best fit for aftermarket wheels is 42mm to 45mm. Opinons may differ some on this, but generally speaking, these suggested offsets are the rule, not the exception.
Usually wheels are stamped on a the back side of the wheel with the offset figure.
Also, be careful buying used wheels, as it is not uncommon to find slight bends. Just something to pay attention to when picking them up.