Thanks for the tip BoingTennis.
I couldn't find the Wilson "Trainer" ball at any of Wilson's international websites. Japan's version does show bags of a "00" pressureless practice ball. Also, some European tennis shops as well as TW Europe are selling a Wilson "Team" trainer ball for 85 Euros. With shipping to the US, that would be pricier than Micro-X from any US retailer.
I did a search on "pressureless balls" in this forum and there seem to be enough satisfied users of the Micro-X so I ordered a bag.
That said, I noticed Tretron has a "Micro-X Comfort" ball which Tretorn's website says is suited to cold climates or high-altitude play. I couldn't find any US vendors of bags however.