If I understand what you are describing, those are the caliper pistons.

I believe with most vehicles they can be replaced separately as part of a brake rebuild kit.

Another thing you will want to watch out for is pressing the brake pedal when the caliper is off of the rotor. That can make the pistons come out completely

Not sure if this is what you need but you can give these guys a call. On the website, look up your year, etc and check under brakes. I think the "wheel cylinder" is what you need.

web page

I hope that helps!