Where to connect C wire to?

Hello, I have a Honeywell Truesteam Humidifier and THM5421R EIM. I am removing the piece of junk Truesteam and replacing with an Aprilaire 800. My question is, do I need to use the Aprilaire Humidistat or can I wire directly from the J12-1 and J12-2 to the EIM U-2 directly? If so, do I need an isolation relay? Also, what functionality will I lose by not using the Aprilaire humidistat?

I have electric baseboard heaters at our cabin which are controlled by an old style mercury dial thermostat. The heaters are wired to 240 Volts but there's a 24 volt relay which allows to use the low voltage stat. I would like to use the Honeywell rth6580wf stat which requires the "C" wire. It says on the package that this thermostat is not compatible with electric baseboard heat (120 -240 V), however, I figured that since a low voltage mercury stat works now, I may be able to use a "smart" thermostat if I can get around the "C" wire issue and power the unit some other way. I was thinking to buy a 110 to 24 V transformer and simply plug the stat into a wall outlet as a way to power it. Alternately I was wondering if I can get get a different relay that comes with a "C" wire. Thanks in advance !