When I have this type of case I look first to the design code, then to ASME II and if not found to Internet. Last item sounds bad but normally I am able to find steel mill who is supplying the material and they normally have published data for their special materials. Other good places are well known suppliers like
www.specialmetals.com. Thermal expansion depends on chemical composition. Small error in values will not "kill" your calculations. Using values for similar material will give you sufficiently accurate result.
This specific steel seems to be 19/18 (Cr/Ni) with 6% Mo. Molybdenium should lower the thermal expansion coefficient so if you would use values for lower Mo steel you would be concervative.