Help with Subwoofer

I have a subwoofer speaker that's rated 500 watts 4 ohms (nominal). I have an amplifier with rated 500 watts rms but requires 24 to 30 ohms speaker. Can't find such a subwoofer. Can I put two speakers with 8 ohms in series to at least give a 16 ohms? Or can I just put a resistor to increase the resistance of the 4 ohms speaker?
 
You could put speakers in series, this will also lower the amount of power each driver needs to handle.

If you want a single driver to take all the power you will need an impedance matching transformer of sufficient size for the amount of power.

Normally these amps with high impedance ratings are designed to be used with speakers that have a built-in matching transformer. Essentially this reduces losses over long runs of speaker wire, something typical of permanent PA system installations (at least that's what I could infer from toying around with such systems).