I am assuming the PSII launch controller just uses one of a variety of premade piezoelectric buzzers available. I installed in an Electro-Launch for school launches, since the buzzer gets everyone's attention, whereas a continuity light does little for a big group of school kids out on the playground.
The buzzer I bought was from Radio Shack (many years ago -- not sure they are still open ...). It is housed in a small round plastic enclosure, with leads protruding from the bottom -- suitable for panel mounting. I ground down the raised area on the controller around the bulb, and epoxied the buzzer in place over the bulb opening. It is wired into the launcher in place of the bulb, soldering each lead to one of the two contacts for the bulb. I do not recall the current draw, but it is pretty low (under 50mA), and is therefore safe for low-current igniters. A Google search shows many piezo buzzers that operate on voltages from about 3V to 15V, and from 10mA to 50mA or so), so there are a lot of buzzers available that are well suited to handling continuity checks safely.
Another mod I made was to ditch the 4 AA batteries inside the controller, and add leads for an external 12V gel cell battery. This controller will fire igniters all day, and was especially handy for group launches. We would typically launch around 100 rockets on the playground each outing. I never worried about the battery power running down.
If I were to make another launcher today, I'd substitute a 3S lipo battery (nominally 11.1VDC) due to their smaller size and lighter weight, but a gel cell battery is a great launch battery for dependable, long-lasting power.