Alesis brand and aftermarket power supplies are available. I bought a Midiverb II when they first came out and still have it. The original supply was 9.5 VAC, 850 mA. There was also a 10 VAC 750 mA and a 9 VAC, 1 amp. As you can see they’re really all the same and all called model P3 by Alesis.
My Midiverb II has a barrel jack, but I’ve heard there were some made with mini-phone jacks, like a mini headphone jack. In that case you would need the model P2. They are the same voltage as the P3, just different jack.
Either of the ones in the links bellow will work for the barrel style, which is the most common.