Andre, I assume you've got an answer to this question by now; please tell us how you solved it! – NauticalMile
Unfortunately I didn't found any information regarding the high temperature values for the viscosity nor the conductivity for those gases. I used a very crude method to get around the issue. Basically, I interpolated the available data with some function up to the upper temperature limit of the available data (usually around 1000K), and then used the obtained functions to extrapolate up to 2000K. The functions where selected on the basis that "they maintained the general tendency on the behavior of the available data up to the maximum limit of 2000K, not allowing for any sharp inflexions or other oscillatory phenomenon". This method is extremely crude, but my application did not require very high precision. You can read it further in my Master's thesis /where I used this information). When the document is available to the public I'll post the link here. Bellow is a figure of the used equations and the available data vs. interpolating/extrapolating functions. Note some discontinuity where the data from NIST (full lines) meet the extraplations (dashed lines):
