Stephan,
I guess a 2.5" plug won't fit your camera...
Kidding aside, I'd recommend the Vello IR remote control, see link:
Vello IR-P1 Infrared Remote Control for Select Pentax IR-P1 B&H
Its battery is the cheap and widely available coin type CR2025, which will last for around 3 years our 20000 releases (according to the remote's specs). I am using it since more than a year now on the same battery and it has seen/been operated in different climate zones from -10 to +45 degrees without any problem. Highly recommended. And it is cheap... around 8 USD. And another plus, the battery can be changed by yourself.
Regards,
Udo