I would like to order some wire online but don't know what it's called. Two-conductor shielded copper wire for 24V, insulated so it could be secured with zip-ties to the HX supply and return pipes running between the boiler and the indirect water heater. It connects to the aquastat. What's this kind of wire called?
18/2 or 16/2 shielded CMR will do
One lesser-known thing about the NEC is that type CM (communications) cables with the correct wire gauge and count can be freely used for any application that calls for a type CL2 or CL3 (low voltage control) cable. So, I'd use a shielded two-conductor in-wall audio or security-alarm cable for this -- these are type CM(R), and many of them are of sufficient gauge to handle anything a thermostat cable can.