You probably don't truly need waterproof switches. The stock switches were not and most have lasted 40 years and seen weather. It is likely possible to refurbish your current momentary switch with a bit of gentle care. Then, if you desire, you can "tin" the ends of the wiring and apply a slight dab of dielectric grease, and cover the bulk of the contact area with shrink wrap to help insulate from the weather. You might also consider the mounting location and orientation of the current momentary to better protect it from gathering moisture.
If you get caught in a downpour while riding, simply wipe the bike down and allow it to dry out afterwards in a warm, cozy little haven.