I have damaged the rim of my rear Shimano RS21 wheel (unexpectedly hit a sharp kerb) which affects braking quite severely so I was wondering if it is possible to replace just the rim and not the whole wheel (the rest is in perfect condition). I've searched a number of online shops and none seem to offer Shimano rims, do Shimano not sell them separately?
You don't have to replace it with the exact same rim. Your wheel doesn't appear to be anything exotic, so anything with the same diameter (700/622 bsd) and the same number of spokes (20?) should be compatible. You'll also probably want to look for something that is about the same width, although you can make small changes to this depending on your needs and availability.