I would think a circulator at 100 MHz would be hard to come by. I have only seen them used in microwave frequencies.
They are commonly used in VHF and UHF repeaters, but I've no idea where you'd get them from, and I imagine they are VERY expensive, and also tuned to a specific frequency.
Not a solution to a simple radio
Everyone is also assuming it's radiation out the front end, there will also be radiation, and more of it, directly from the LO itself.
As he's only wanting to simulate it anyway, how accurately do simulators simulate stray radiation? - do they at all?.