There are two good sources for Redline MT-90 which is a great choice for the AX-5 and AX-15, Summit Racing at Summit Racing
as suggested above and Redline Oil at Red Line Synthetic Oil
Summit's price will be substantially cheaper than Redline's full MSRP price.
Or, simply use a good synthetic 10W-30 engine oil in your AX-5 which was approved for use in the AX-5 and AX-15 by Aisin, the transmission manufacturer, a few years ago. 10W-30 engine oil is about the same viscosity as the 75W-90 MT-90 gear lube since they are measured on two different viscosity sales.