You can buy purely mechanical versions with adjustable resistance. I got one a couple of years ago.

Breathing sounds easy but it is real exercise if you set the resistance high enough. I found it unpleasant and stopped doing it, especially because the device gets kind of gross and needs frequent disassembly for washing according to the manual. But I really should probably start again given the low time investment and multiple studies that show a large blood pressure benefit relative to other interventions.