The service manual describes that the piston can't be removed from the rear caliper, so there isn't really a full rebuild option. Looks like you can replace the piston boot, & the bushing in the mounting ears, & the p-brake o rings if you separate the motor from the caliper. They also show a special tool to install the boot, unsure if that's truly necessary? Boot diagram & part number here

 

I managed to get a replacement boot for the rebuild. It has a metal ring built into one side of the boot.

I'm a bit confused about the warnings in the Air Bleeding section of the service manual:
CAUTION:
• For the caliper with electronic parking actuator, never drain the brake fluid in the caliper.
• When removing the caliper with electronic parking actuator from the vehicle, make sure to apply the brake fluid seal before disconnection.
• Do not remove the brake fluid seal of the caliper with replacement electronic parking actuator, except when the caliper is installed on the vehicle.

This is a rear caliper with electronic parking actuator. How am I supposed to not drain the brake fluid if I'm going to change the boot? It's all gonna gush out when I pull the old boot off, and then I'll have to refill and air bleed it after installing the replacement boot.

 

I haven't had to mess with any calipers on my 18 yet so my only experience is older traditional calipers. Replacement of the boot isn't going to dump out brake fluid that requires refilling & bleeding. The dust boot just keeps the piston clean as it extends during operation or as the pads wear, it's not a fluid seal.

My read of the PDF I posted is Subaru saying to replace the bushing and/or boot with the caliper unbolted but the fluid line still attached, so you don't even have to mess with air bleeding. The actual fluid seal- internal around the piston- is not touched. A full rebuild of an older style caliper includes removing the piston & replacing that seal, but the service manual says that's not possible with this caliper.

Obviously it's somehow possible since you can buy rebuilt calipers, but it's not meant to be field serviced & as you found, they don't sell the internal seal.

 

I managed to do this on the weekend. Wasn't bad at all. Here are some tips:

1) the replacement dust boot has a metal ring inside it. Don't flex it and end up bending that ring.

2) push the piston all the way in.

3) with a screwdriver, get the old boot off.

4) lube the new dust boot with some good stuff. I used permatex 24129 rubber compatible grease. Dealer mech says he justs uses some brake fluid.

4) hard part: put the new (slippery) boot on. part of where the boot seats on the caliper is hard to reach, and you won't get your fingers in there, so that is where you want to start. then you work around to the other side to finish seating it. My fingers couldn't manage to get it on, and I was afraid to use a screwdriver.

5) The subaru tool is unavailable to us mere mortals. I figured out that if I use the cap of shaving cream can, that is the perfect diameter. I had an "Edge" brand shaving can with a flat top, roughly 2in diameter, but I think they're all standardized. I used a brake caliper tool to push the cap onto the boot until it fully seated around the caliper. The sharp edge of the cap isn't so sharp that it cuts into the rubber boot. The cap is a bit flexible so it's better to compress with a caliper tool to spread the pressure, rather than use a screwdriver that could cause the cap to fold (like I did on first attempt).

6) pump the brake pedal a bit to get the piston out about a quarter inch. That gets the piston groove out enough for you to seat the other end of the boot around the piston in that groove. That part is much easier than the larger diameter ring.

7) Since I noticed a little brake fluid coming out of the torn boot, I was nervous I would need to bleed the brakes after the replacement. But the internal piston seal ring is still there, and no air enters the brake system. I think maybe when the piston came out all the way some fluid escaped into the boot.

Definitely a doable job if you ever end up with a tear. I called a few dealership mechs to see if they knew how to do it, and all but one said they've never done it, they always just replace the whole caliper with a rebuilt one. Bloody "professionals". This was a 10 dollar repair and I sitll have the original OEM grade caliper.