Would members who are fire safety trainers have any advice on self refillable training fire extinguishers typically water and Co2 as less mess. Leader, Jockel, Gloria, Helimlick make models specific for training but I hear of others using Amerix 240 model or similar with a schrader valve for pressurizing. I would be interested to hear how other trainers manage it in a cost effective way.  Thank you

Low pressure (100psi, so standard airline or compressor can be used) stainless steel water extinguishers with built in Schrader valve used to be very common before 2000 with UK made Chubb or US made Badger & Amerex models being around in volume. The post 1997 move to the trade becoming disposable coupled with the 'bin it if not red' purge of 97-99 (when we moved away from fully colour coded bodies) means hardly any are left about and with the move to cheap Chinese extinguishers they are not readily obtainable new.
I have a small stock as I deliberately went out of my way to save them whenever I came across them and do sell some of them off from time to time as I have more than enough for my own needs.
As a lot of perfectly good extinguishers are scrapped at 5 years as hardly anyone discharge tests and refills these days many trainers will buy these surplus water and CO2 extinguishers from service companies at a far reduced price for single use. I have loads available for trainers to buy, as it seems a waste to scrap full extinguishers.
If you do enough volume of training then buying a stock of new stainless steel waters, new CO2 and the head cap bars, filling adapters & Nitrogen kit ends up cheaper in the long run