Any recommendations for training gear for those of us living at 2,500 ft. to help us aclimatize better to 8,000 to 12,000 ft.?

I recall there used to be an expensive, cumbersome and stupid-looking device that made you rebreathe some of your CO2 to make for "thin air" and replicate different altitudes, depending on adjustment. Maybe this would be OK on an exercise bike stand in the privacy of your own house but never would I be caught wearing it in public.

Eric
P.S.
I ask this B/C, when climbing to almost 12,000 ft. last Sunday I was forced to walk 1/2 as fast as my normal pace at 2,000 feet.