Most heat transfer oil companies can be found easily by doing a web-based search on "heat transfer fluids."  I would offer caution regarding the mixing of different brand fluids without a thorough draining, cleaning and flushing of the system because of the risk of incompatible materials.  If you are switching suppliers, then the new supplier should be able to offer guidance on the proper methods for replacing the existing oil.  If the new supplier cannot do this, then I would always recommend a thorough drain/clean/flush approach.