What is a momentary SPST Push Button Switch?
SPST NO (Normally Open) Momentary switch is exactly what is inside a computer keyboard - press for on, let go for off.

It sounds like you are looking for a Latching switch (works like a ballpoint pen) instead of Momentary.  

If you can only find one with more poles or features (DPDT) it'll do the job - just ignore the extra features.

A brief look shows this from the same seller, I suspect he has others...