Agree with both the above posters. Providing the caliper bores are not too far gone then rebuild kits with replacement pistons (do both sides to avoid having to do much of the work again later - such as waiting for a dry day, jacking up and crawling underneath, bleeding the whole system etc.).
The cause of the corroded pistons is moisture in the brake fluid and is why the fluid should be replaced every few years. This is also to avoid brake failure, where the heat of continous braking builds up and the moisture in the fluid boils and becomes a gas and then compresses and you "run out of brake pedal" and it hits the floor.
I believe BiggRed (as mentioned above) supply rebuilt calipers and if they can that would be the way to go if you don't fancy rebuilding yours, although they do appear to stock a wide variety of various rebuild kits.