Shifter/brake-lever compatibility with front and rear derailleurs
You can actually use any 8, 9 or 10 speed Shimano rear road derailleur, or an 8 or 9 speed rear mountain derailleur with your 8 speed shifters and cassette.

The reason for this is that Shimano used the same actuation ratio (ratio of lateral movement to length cable pulled) for all those derailleurs. As long as the shifters match the cassette it will work.

There is one exception I know of which is Tiagra 4700 10 speed, which uses Shimano's 11 speed actuation ratio (See Compatibility of bar end Shifters and 10 spd Tiagra).