Well, in case of TTL output i.e. 5V, you can also use a voltage Regulator IC which is LM7805 .. It gives constant 5V at output and input range is also upto 25V to 30V .. Read datasheet of lm7805 as well ..
Please specify purpose like switching speed, direction, current source and sink capabilities, etc.
Analog Devices have a very good range of isolators. Link