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?.
that would require replacing the existing tuning cap with one with an extra tuning gang with the same capacitance as the LO. not exactly an off-the-shelf solution. a proper mixer, however can be an off-the-shelf fix, since there are companies that make them as modules. they are wideband modules, so nothing special needs to be done to the radio they're going in to, except some impedance matching.