What is the best instrument amplifier to measure microvolt dc signals from a low impedance differential type T thermocouple system?
Use a &quot;zero-drift&quot; amplifier with as low noise as possible and acceptable drift. You are looking for about 800nV difference, so pick something with low DC-10Hz or DC-1Hz peak-to-peak noise. Your source will be low impedance so you don't need to worry too much about current noise.
A regular op-amp will be better than an instrumentation amplifier most likely depending on grounding and such like, you should only use an instrumentation amplifier if you have no other options.
Looking at Digikey, I see around 100 options, you can either start looking at datasheets individually, or go to manufacturer websites where they may include noise in the parameters. There are only a few manufacturers of suitable products so that's not as big a burden as it might seem. The cheapest listed is the LTC2054 with 30nV/°C maximum drift and typical (no maximum) 600nVp-p 0-1Hz noise.
Beyond the scope of this answer, but you will have to be exquisitely careful to minimize thermal EMFs and thermal gradients to get useful performance at this level. Read any and all application notes you can find before beginning construction. Sub-microvolt DC measurements are closer to a physics experiment than routine electronics design.