Is 'Spartan Sleeper' still available? Or are there better ones now? Friends in Camp Verde, AZ had good results with that one about 20 years ago.

Most main crop onions will store very well and the conditions under which you harvest, dry and store them is far more important than variety.
Another approach is to make sure you are cultivating spring, summer and autumn types, including Japanese onions planted in autumn that will be mature in May or June. That way you can have onions all year round.