Hi there are lots of replacement shocks out there google will find them. If you are just going to replace the rears I would go with original specification parts. Using a loaded part, which will be stiffer, just on the back will change the balance of the car in unpredictable way.
For example on hard breaking in a corner you typically want the rebound rate on the rear to be slightly faster than the front so the rear wheels reamain in good contact with the road and you don't end up spinning.
If you are going to change all of them then use a set from one manufacturer, unless you really know what you are doing in suspension tuning this is much safer.