Here’s what I’ve found out…
Alesis customer support have said in three different posts that “Any momentary, non-latching footswitch” will work.
I’m going to add to this that the switch has to be able to use a TS cable (regular instrument cable) and not a TRS cable (these have a second wire, like a stereo headphone cable).
I found three options that will work.
This one was designed for people that want to do exactly what you want to do. Specifically, it was made with the SR-16/SR-18 in mind. Drum Machine Footswitch $50
I don’t know anything about this company and I can’t make a recommendation one way or the other regarding this pedal. I think it’s interesting that since it exists at all, others have also wanted the same solution you’re looking for.
Next is this… Fender 1-Button Footswitch $35 each
I could swear I saw somewhere that this was a non-latching footswitch but I can’t find it now. I saw in one of your pictures that you have one. You could test it with the SR-18 and verify whether it works or not. I would sure like to know if you have a chance to test it.
The only downside is the connection is on the side which be cumbersome on your pedalboard if you decide to add things later.
Last is this… Boss FS-5U $40 each
These are designed to attach together and have the connection on the top edge. If money wasn’t a factor I would choose this one. But with the cost of cables we’re looking at $120 solution versus the $26 solution you have now.
As an alternative, if you want to minimize your cabling, you could do this…
Nux Dual Footswitch $40
To TRS cable $14
To TRS female to 2 TS male adapter $6
To the two connections on the SR-18
Based on the what Nux says, if you set the switch to ‘close’ then the switch will work non-powered and by using the TRS to TS adapter you get around the Alesis not accepting a TRS connection. Also, it would take up less space on your pedal board and reduce cabling. And after checking, it’s the least expensive solution I’ve brought up. This is what I would go with.
Good luck, Pam. Let me know if you have any questions.