tried e bay? scrap yard? you can ask online.

Don't know your particular car but if it operates the handbrake as well it will probably need screwing in. If not you should be able to free it off by levering it back into the cylinder with a large tyre lever, you will need to remove the brake pads to do this. Gently pull back the rubber seal round the edge and pour some brake fluid into it, then lever it as far as it will go and get someone to put their foot on the brake to push it out again, when you have done this a few times it should free off. Lever it squarely or it will jam.

took me less than 20 seconds to find this:

www.ebay.co.uk/itm/Rear-Right-Brake-Caliper-To-Fit...c

Rear caliper pistons usually screw back in. Sometimes clockwise, sometimes counterclockwise. Consult your Haynes manual.
You can sometimes do it with long nose pliers or similar, but wind back tools are cheap. I have a universal set from an eBay shop that cost £28 and it seems fine.

TBH, I'd go for the new unit. And do both sides. You could end up with the rear brakes out of balance which is dangerous and may be an MoT failure. Either that or get your rear brakes tested at your local friendly MoT station once you've done the faulty side.