Like KennyRodgers said, Rock Auto is a good place to get parts in general. I get stuff from there when I travel to the States. Sometimes you have to know the manufacturer part number you are looking for rather than going by their vehicle menu. I did notice that Rock Auto did not have the control arms for the MH P30 chassis, only regular duty.
With the motorhome version of the P30, you will be looking for parts for the heavy duty suspension. This is where the rear disc brakes come in. The regular duty P30 suspension had drums in the rear. The discs in the back indicate beefier control arms. Some parts are listed as motor home P30 specific when you look at the manufacturers' websites. When you start searching for P30 control arms, you realize that there are a few different ones depending on the suspension setup.
You might also try checking out stepvanparts.com
. They specialize in the P30 vans have good diagrams of different P30 parts and may be able to help you out.
Is it all 4 control arms or just the uppers or lowers?