I suspect your Opto isolator has an LED that has a voltage drop greater than 3.3 volts, its probably 3.5 -3.8 volts.
As far as the output side just make sure its designed for the type of current, AC or DC, and has a voltage rating of at least 2x you r relay voltage. Sam with the current capacity.
All of the big electronics components dealer carry them on-line so finding one with the right specs and operating ranges and with built in snubber is not hard.
Digikey, Mouser and often times eBay have loads of SSR devices.