Where in the system is the leak? One joint? Multiple joints? Cartridge?
If the O-rings are non standard you can make your own. Kits are available ( we used them in our factory).
You buy a length of correct diameter material, cut it to length and glue the ends together. Bingo....a new O-ring. I made a couple last year for a hose fitting. There were none left in my box of O-rings so ordered material ( pay per metre) and made a couple. Have a look on you tube for “how to” vids.
Re the no isolator valve, maybe get a plumber in to fit one. Trace the feed back to somewhere you can fit it, even cutting a hole for access and repairing/ fit a panel.
