Ebay?None on UK site at moment but there are some on ebay.de
Theres nothing you can really do to prevent them sticking(at least i dont think)
The most common reason for them to stick in first place is that dust and dirt get past the rubber seals around the piston,forming rust.Meaning the caliper cant move smoothly within its cylinder and so sticks.
If once you get them loosened up and cleaned they still keep sticking,you may think about getting them rebuilt(can be expensive) or buying a new set(expensive) or just getting a set off a car in scrapyard(cheap) i
Spacestar brakes on ebay germany