FindOrder works fine for me in some cases... for example, if I look for OrderType. I get the error if I include OrderSource in the request. Have you tried this? BTW I'm working with the version 7.0.3 on my windows 7 of 64 bits.
it could be a bug.
Anyway, what JP says is absolutely true: in any 7.* cartridge, order type and order source are populated with one and the same value; which is ppresented as Order in the design studio.
So just replace the order source criteria with the order type criteria with the same values and you should be ok.